PowerPC: Detect AltiVec support on OS X

libjpeg-turbo's AltiVec SIMD extensions previously assumed that AltiVec
instructions were available on all Power Macs that supported OS X 10.4
"Tiger" (the earliest version of OS X that libjpeg-turbo has ever
supported), but Tiger can actually run on PowerPC G3 processors, which
lack AltiVec instructions.  This commit enables run-time detection of
AltiVec instructions on OS X/PowerPC systems if AltiVec instructions are
not force-enabled at compile time (using -maltivec).  This allows the
same build of libjpeg-turbo to support G3, G4, and G5 Power Macs.

Closes #609

ChangeLog.md

@@ -12,6 +12,11 @@ Visual Studio 2010.
 used to prime the decompressor with quantization and Huffman tables that can be
 used when decompressing subsequent "abbreviated image" datastreams.
 
+3. libjpeg-turbo now performs run-time detection of AltiVec instructions on
+OS X/PowerPC systems if AltiVec instructions are not enabled at compile time.
+This allows both AltiVec-equipped (PowerPC G4 and G5) and non-AltiVec-equipped
+(PowerPC G3) CPUs to be supported using the same build of libjpeg-turbo.
+
 
 2.1.3
 =====

simd/powerpc/jsimd.c

@@ -29,7 +29,10 @@
 
 #include <ctype.h>
 
-#if defined(__OpenBSD__)
+#if defined(__APPLE__)
+#include <sys/types.h>
+#include <sys/sysctl.h>
+#elif defined(__OpenBSD__)
 #include <sys/param.h>
 #include <sys/sysctl.h>
 #include <machine/cpu.h>
@@ -119,6 +122,10 @@ init_simd(void)
   int bufsize = 1024; /* an initial guess for the line buffer size limit */
 #elif defined(__amigaos4__)
   uint32 altivec = 0;
+#elif defined(__APPLE__)
+  int mib[2] = { CTL_HW, HW_VECTORUNIT };
+  int altivec;
+  size_t len = sizeof(altivec);
 #elif defined(__OpenBSD__)
   int mib[2] = { CTL_MACHDEP, CPU_ALTIVEC };
   int altivec;
@@ -132,7 +139,7 @@ init_simd(void)
 
   simd_support = 0;
 
-#if defined(__ALTIVEC__) || defined(__APPLE__)
+#if defined(__ALTIVEC__)
   simd_support |= JSIMD_ALTIVEC;
 #elif defined(__linux__) || defined(ANDROID) || defined(__ANDROID__)
   while (!parse_proc_cpuinfo(bufsize)) {
@@ -144,7 +151,7 @@ init_simd(void)
   IExec->GetCPUInfoTags(GCIT_VectorUnit, &altivec, TAG_DONE);
   if (altivec == VECTORTYPE_ALTIVEC)
     simd_support |= JSIMD_ALTIVEC;
-#elif defined(__OpenBSD__)
+#elif defined(__APPLE__) || defined(__OpenBSD__)
   if (sysctl(mib, 2, &altivec, &len, NULL, 0) == 0 && altivec != 0)
     simd_support |= JSIMD_ALTIVEC;
 #elif defined(__FreeBSD__)