Support architectures where `long` is smaller than pointers.

I am currently trying to compile FreeType for CHERI-extended ISAs
(CHERI-RISC-V and Arm's Morello), but I am getting compiler warnings
from the `FT_UINT_TO_POINTER` macro.  When compiling with the CHERI
Clang compiler, not using `uinptr_t` for casts between integers an
pointers results in the following `-Werror` build failures:

```
In file included from .../src/truetype/truetype.c:22:
  .../src/truetype/ttgload.c:1925:22: error:
    cast from provenance-free integer type to pointer type will
    give pointer that can not be dereferenced
    [-Werror,-Wcheri-capability-misuse]
  node->data = FT_UINT_TO_POINTER( glyph_index );
               ^
  .../include/freetype/internal/compiler-macros.h:79:34: note:
    expanded from macro 'FT_UINT_TO_POINTER'
```

* include/freetype/internal/compiler-macros.h (FT_UINT_TO_POINTER):
The ISO C standard compliant fix for this would be to use
`uintptr_t` from `stdint.h`, but I am not sure if this is supported
by the minimum compiler version.  Therefore, use the
compiler-defined `__UINTPTR_TYPE__` macro (supported in GCC 4.6+ and
Clang since about 3.0) before checking for `_WIN64` and falling back
to `unsigned long`.
diff --git a/ChangeLog b/ChangeLog
index c8a0795..751fdd4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,33 @@
+2021-07-15  Alex Richardson  <Alexander.Richardson@cl.cam.ac.uk>
+
+	Support architectures where `long` is smaller than pointers.
+
+	I am currently trying to compile FreeType for CHERI-extended ISAs
+	(CHERI-RISC-V and Arm's Morello), but I am getting compiler warnings
+	from the `FT_UINT_TO_POINTER` macro.  When compiling with the CHERI
+	Clang compiler, not using `uinptr_t` for casts between integers an
+	pointers results in the following `-Werror` build failures:
+
+	```
+	In file included from .../src/truetype/truetype.c:22:
+	  .../src/truetype/ttgload.c:1925:22: error:
+	    cast from provenance-free integer type to pointer type will
+	    give pointer that can not be dereferenced
+	    [-Werror,-Wcheri-capability-misuse]
+	  node->data = FT_UINT_TO_POINTER( glyph_index );
+	               ^
+	  .../include/freetype/internal/compiler-macros.h:79:34: note:
+	    expanded from macro 'FT_UINT_TO_POINTER'
+	```
+
+	* include/freetype/internal/compiler-macros.h (FT_UINT_TO_POINTER):
+	The ISO C standard compliant fix for this would be to use
+	`uintptr_t` from `stdint.h`, but I am not sure if this is supported
+	by the minimum compiler version.  Therefore, use the
+	compiler-defined `__UINTPTR_TYPE__` macro (supported in GCC 4.6+ and
+	Clang since about 3.0) before checking for `_WIN64` and falling back
+	to `unsigned long`.
+
 2021-07-13  Oleg Oshmyan  <chortos@inbox.lv>
 
 	[base] Fix `FT_Open_Face`'s handling of user-supplied streams.
diff --git a/include/freetype/internal/compiler-macros.h b/include/freetype/internal/compiler-macros.h
index 0eb5e24..d8b61b3 100644
--- a/include/freetype/internal/compiler-macros.h
+++ b/include/freetype/internal/compiler-macros.h
@@ -71,12 +71,18 @@
    */
 #define FT_DUMMY_STMNT  FT_BEGIN_STMNT FT_END_STMNT
 
-#ifdef _WIN64
+#ifdef __UINTPTR_TYPE__
+  /*
+   * GCC and Clang both provide a `__UINTPTR_TYPE__` that can be used to
+   * avoid a dependency on `stdint.h`.
+   */
+#  define FT_UINT_TO_POINTER( x )  (void *)(__UINTPTR_TYPE__)(x)
+#elif defined( _WIN64 )
   /* only 64bit Windows uses the LLP64 data model, i.e., */
   /* 32-bit integers, 64-bit pointers.                   */
-#define FT_UINT_TO_POINTER( x )  (void *)(unsigned __int64)(x)
+#  define FT_UINT_TO_POINTER( x )  (void *)(unsigned __int64)(x)
 #else
-#define FT_UINT_TO_POINTER( x )  (void *)(unsigned long)(x)
+#  define FT_UINT_TO_POINTER( x )  (void *)(unsigned long)(x)
 #endif
 
   /*