ARM/MIPS: Change the behavior of JSIMD_FORCE*

The JSIMD_FORCE* environment variables previously meant "force the use
of this instruction set if it is available but others are available as
well", but that did nothing on ARM platforms, since there is only ever
one instruction set available.  Since the ARM and MIPS CPU feature
detection code is less than bulletproof, and since there is only one
SIMD instruction set (currently) supported on those platforms, it makes
sense for the JSIMD_FORCE* environment variables on those platforms to
actually force the use of the SIMD instruction set, thus bypassing the
CPU feature detection code.

This addresses a concern raised in #88 whereby parsing /proc/cpuinfo
didn't work within a QEMU environment.  This at least provides a
workaround, allowing users to force-enable or force-disable SIMD
instructions for ARM and MIPS builds of libjpeg-turbo.
diff --git a/ChangeLog.md b/ChangeLog.md
index 0691ccc..9653471 100644
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,3 +1,26 @@
+1.5.1
+=====
+
+### Significant changes relative to 1.5.0:
+
+1. Previously, the undocumented `JSIMD_FORCE*` environment variables could be
+used to force-enable a particular SIMD instruction set if multiple instruction
+sets were available on a particular platform.  On x86 platforms, where CPU
+feature detection is bulletproof and multiple SIMD instruction sets are
+available, it makes sense for those environment variables to allow forcing the
+use of an instruction set only if that instruction set is available.  However,
+since the ARM implementations of libjpeg-turbo can only use one SIMD
+instruction set, and since their feature detection code is less bulletproof
+(parsing /proc/cpuinfo), it makes sense for the `JSIMD_FORCENEON` environment
+variable to bypass the feature detection code and really force the use of NEON
+instructions.  A new environment variable (`JSIMD_FORCEDSPR2`) was introduced
+in the MIPS implementation for the same reasons, and the existing
+`JSIMD_FORCENONE` environment variable was extended to that implementation.
+These environment variables provide a workaround for those attempting to test
+ARM and MIPS builds of libjpeg-turbo in QEMU, which passes through
+/proc/cpuinfo from the host system.
+
+
 1.5.0
 =====
 
diff --git a/simd/jsimd_arm.c b/simd/jsimd_arm.c
index 754806d..61cd073 100644
--- a/simd/jsimd_arm.c
+++ b/simd/jsimd_arm.c
@@ -125,7 +125,7 @@
   /* Force different settings through environment variables */
   env = getenv("JSIMD_FORCENEON");
   if ((env != NULL) && (strcmp(env, "1") == 0))
-    simd_support &= JSIMD_ARM_NEON;
+    simd_support = JSIMD_ARM_NEON;
   env = getenv("JSIMD_FORCENONE");
   if ((env != NULL) && (strcmp(env, "1") == 0))
     simd_support = 0;
diff --git a/simd/jsimd_arm64.c b/simd/jsimd_arm64.c
index 7def8f9..09449bb 100644
--- a/simd/jsimd_arm64.c
+++ b/simd/jsimd_arm64.c
@@ -142,7 +142,7 @@
   /* Force different settings through environment variables */
   env = getenv("JSIMD_FORCENEON");
   if ((env != NULL) && (strcmp(env, "1") == 0))
-    simd_support &= JSIMD_ARM_NEON;
+    simd_support = JSIMD_ARM_NEON;
   env = getenv("JSIMD_FORCENONE");
   if ((env != NULL) && (strcmp(env, "1") == 0))
     simd_support = 0;
diff --git a/simd/jsimd_mips.c b/simd/jsimd_mips.c
index 358bbb8..63b8115 100644
--- a/simd/jsimd_mips.c
+++ b/simd/jsimd_mips.c
@@ -2,7 +2,7 @@
  * jsimd_mips.c
  *
  * Copyright 2009 Pierre Ossman <ossman@cendio.se> for Cendio AB
- * Copyright (C) 2009-2011, 2014, D. R. Commander.
+ * Copyright (C) 2009-2011, 2014, 2016, D. R. Commander.
  * Copyright (C) 2013-2014, MIPS Technologies, Inc., California.
  * Copyright (C) 2015, Matthieu Darbois.
  *
@@ -77,6 +77,14 @@
   if (!parse_proc_cpuinfo("MIPS 74K"))
     return;
 #endif
+
+  /* Force different settings through environment variables */
+  env = getenv("JSIMD_FORCEDSPR2");
+  if ((env != NULL) && (strcmp(env, "1") == 0))
+    simd_support = JSIMD_MIPS_DSPR2;
+  env = getenv("JSIMD_FORCENONE");
+  if ((env != NULL) && (strcmp(env, "1") == 0))
+    simd_support = 0;
 }
 
 static const int mips_idct_ifast_coefs[4] = {