[autofit] Split `afwrtsys.h`.

The header file `afwrtsys.h` has two distinct functions: to include the
required writing system headers and also to generate code for each writing
system.  At each current use site only one or the other is used, with
various macro trickery selecting one or the other.  Split this header into
`afws-decl.h` for the required writing system declarations and `afws-iter.h`
for iterating over the writing systems to generate code.

The motivation for this change is that the Visual C++ compiler treats the
standard include guard idiom like `#pragma once` 'if no non-comment code or
preprocessor directive comes before or after the standard form of the
idiom'.  It appears to check this after macro expansion, so if
`WRITING_SYSTEM` expands to empty the bottom of `afwrtsys.h` is empty and
looks like the standard include guard idiom which is treated like `#pragma
once`, so subsequent inclusion of `afwrtsys.h` is elided.

Fixes #1075.

* src/autofit/afglobal.c (af_writing_system_classes), src/autofit/aftypes.h
(AF_WritingSystem), src/autofit/rules.mk (AUTOF_DRV_H): Updated.

* src/autofit/afwrtsys.h: Split into...
* src/autofit/afws-decl.h, src/autofit/afws-iter.h: New files.
diff --git a/src/autofit/afglobal.c b/src/autofit/afglobal.c
index aedb74d..577b94e 100644
--- a/src/autofit/afglobal.c
+++ b/src/autofit/afglobal.c
@@ -19,6 +19,7 @@
 #include "afglobal.h"
 #include "afranges.h"
 #include "afshaper.h"
+#include "afws-decl.h"
 #include <freetype/internal/ftdebug.h>
 
 
@@ -32,11 +33,6 @@
 #define FT_COMPONENT  afglobal
 
 
-  /* get writing system specific header files */
-#undef  WRITING_SYSTEM
-#define WRITING_SYSTEM( ws, WS )  /* empty */
-#include "afwrtsys.h"
-
 #include "aferrors.h"
 
 
@@ -74,7 +70,7 @@
   af_writing_system_classes[] =
   {
 
-#include "afwrtsys.h"
+#include "afws-iter.h"
 
     NULL  /* do not remove */
   };
diff --git a/src/autofit/aftypes.h b/src/autofit/aftypes.h
index e76b103..1d792b9 100644
--- a/src/autofit/aftypes.h
+++ b/src/autofit/aftypes.h
@@ -198,7 +198,6 @@
    *   outline according to the results of the glyph analyzer.
    */
 
-#define AFWRTSYS_H_  /* don't load header files */
 #undef  WRITING_SYSTEM
 #define WRITING_SYSTEM( ws, WS )    \
           AF_WRITING_SYSTEM_ ## WS,
@@ -207,14 +206,12 @@
   typedef enum  AF_WritingSystem_
   {
 
-#include "afwrtsys.h"
+#include "afws-iter.h"
 
     AF_WRITING_SYSTEM_MAX   /* do not remove */
 
   } AF_WritingSystem;
 
-#undef  AFWRTSYS_H_
-
 
   typedef struct  AF_WritingSystemClassRec_
   {
diff --git a/src/autofit/afwrtsys.h b/src/autofit/afwrtsys.h
deleted file mode 100644
index 39aa121..0000000
--- a/src/autofit/afwrtsys.h
+++ /dev/null
@@ -1,45 +0,0 @@
-/****************************************************************************
- *
- * afwrtsys.h
- *
- *   Auto-fitter writing systems (specification only).
- *
- * Copyright (C) 2013-2021 by
- * David Turner, Robert Wilhelm, and Werner Lemberg.
- *
- * This file is part of the FreeType project, and may only be used,
- * modified, and distributed under the terms of the FreeType project
- * license, LICENSE.TXT.  By continuing to use, modify, or distribute
- * this file you indicate that you have read the license and
- * understand and accept it fully.
- *
- */
-
-
-#ifndef AFWRTSYS_H_
-#define AFWRTSYS_H_
-
-  /* Since preprocessor directives can't create other preprocessor */
-  /* directives, we have to include the header files manually.     */
-
-#include "afdummy.h"
-#include "aflatin.h"
-#include "afcjk.h"
-#include "afindic.h"
-
-#endif /* AFWRTSYS_H_ */
-
-
-  /* The following part can be included multiple times. */
-  /* Define `WRITING_SYSTEM' as needed.                 */
-
-
-  /* Add new writing systems here.  The arguments are the writing system */
-  /* name in lowercase and uppercase, respectively.                      */
-
-  WRITING_SYSTEM( dummy,  DUMMY  )
-  WRITING_SYSTEM( latin,  LATIN  )
-  WRITING_SYSTEM( cjk,    CJK    )
-  WRITING_SYSTEM( indic,  INDIC  )
-
-/* END */
diff --git a/src/autofit/afws-decl.h b/src/autofit/afws-decl.h
new file mode 100644
index 0000000..c10dd57
--- /dev/null
+++ b/src/autofit/afws-decl.h
@@ -0,0 +1,33 @@
+/****************************************************************************
+ *
+ * afws-decl.h
+ *
+ *   Auto-fitter writing system declarations (specification only).
+ *
+ * Copyright (C) 2013-2021 by
+ * David Turner, Robert Wilhelm, and Werner Lemberg.
+ *
+ * This file is part of the FreeType project, and may only be used,
+ * modified, and distributed under the terms of the FreeType project
+ * license, LICENSE.TXT.  By continuing to use, modify, or distribute
+ * this file you indicate that you have read the license and
+ * understand and accept it fully.
+ *
+ */
+
+
+#ifndef AFWS_DECL_H_
+#define AFWS_DECL_H_
+
+  /* Since preprocessor directives can't create other preprocessor */
+  /* directives, we have to include the header files manually.     */
+
+#include "afdummy.h"
+#include "aflatin.h"
+#include "afcjk.h"
+#include "afindic.h"
+
+#endif /* AFWS_DECL_H_ */
+
+
+/* END */
diff --git a/src/autofit/afws-iter.h b/src/autofit/afws-iter.h
new file mode 100644
index 0000000..5571420
--- /dev/null
+++ b/src/autofit/afws-iter.h
@@ -0,0 +1,31 @@
+/****************************************************************************
+ *
+ * afws-iter.h
+ *
+ *   Auto-fitter writing systems iterator (specification only).
+ *
+ * Copyright (C) 2013-2021 by
+ * David Turner, Robert Wilhelm, and Werner Lemberg.
+ *
+ * This file is part of the FreeType project, and may only be used,
+ * modified, and distributed under the terms of the FreeType project
+ * license, LICENSE.TXT.  By continuing to use, modify, or distribute
+ * this file you indicate that you have read the license and
+ * understand and accept it fully.
+ *
+ */
+
+  /* This header may be included multiple times. */
+  /* Define `WRITING_SYSTEM' as needed.          */
+
+
+  /* Add new writing systems here.  The arguments are the writing system */
+  /* name in lowercase and uppercase, respectively.                      */
+
+  WRITING_SYSTEM( dummy, DUMMY )
+  WRITING_SYSTEM( latin, LATIN )
+  WRITING_SYSTEM( cjk,   CJK   )
+  WRITING_SYSTEM( indic, INDIC )
+
+
+/* END */
diff --git a/src/autofit/rules.mk b/src/autofit/rules.mk
index 19b3bca..499c0e0 100644
--- a/src/autofit/rules.mk
+++ b/src/autofit/rules.mk
@@ -42,13 +42,14 @@
 
 # AUTOF driver headers
 #
-AUTOF_DRV_H := $(AUTOF_DRV_SRC:%c=%h)  \
-               $(AUTOF_DIR)/afcover.h  \
-               $(AUTOF_DIR)/aferrors.h \
-               $(AUTOF_DIR)/afscript.h \
-               $(AUTOF_DIR)/afstyles.h \
-               $(AUTOF_DIR)/aftypes.h  \
-               $(AUTOF_DIR)/afwrtsys.h
+AUTOF_DRV_H := $(AUTOF_DRV_SRC:%c=%h)   \
+               $(AUTOF_DIR)/afcover.h   \
+               $(AUTOF_DIR)/aferrors.h  \
+               $(AUTOF_DIR)/afscript.h  \
+               $(AUTOF_DIR)/afstyles.h  \
+               $(AUTOF_DIR)/aftypes.h   \
+               $(AUTOF_DIR)/afws-decl.h \
+               $(AUTOF_DIR)/afws-iter.h
 
 
 # AUTOF driver object(s)