Add configure checks for cpuid symbols. (PR #116)

Fixes #115.

Testing compiler versions is tricky, especially given some vendors
wrap up standard compilers as their own, but change version numbers
(Apple, Intel) and compilers also often masquerade as another (eg
setting GNUC).

Instead check for the __get_cpuid_max and __cpuid_count symbols
in configure, and adjust the code guards to use the results.

Author: jkbonfield

configure.ac

@@ -120,6 +120,9 @@ AS_IF([test "$ax_cv_builtin_prefetch" = "yes"],
            [Define to 1 if you have a __builtin_prefetch])],
 	[])
 
+dnl Check is cpuid works, needed by rANS_static4x16pr.c.
+AC_CHECK_DECLS([__get_cpuid_max, __cpuid_count], [], [], [[#include <cpuid.h>]])
+
 dnl AC_CHECK_LIB([lzma], [lzma_easy_buffer_encode], [
 dnl 	LIBS="-llzma $LIBS"
 dnl 	AC_DEFINE([HAVE_LIBLZMA],1,[Define to 1 if you have the liblzma library.])])

htscodecs/rANS_static4x16pr.c

@@ -824,8 +824,9 @@ unsigned char *rans_uncompress_O1_4x16(unsigned char *in, unsigned int in_size,
 
 static int rans_cpu = 0xFFFF; // all
 
-#if (defined(__GNUC__) || defined(__clang__)) && defined(__x86_64__)
-// Icc and Clang both also set __GNUC__ on linux, but not on Windows.
+#if defined(__x86_64__) && \
+    defined(HAVE_DECL___CPUID_COUNT)   && HAVE_DECL___CPUID_COUNT && \
+    defined(HAVE_DECL___GET_CPUID_MAX) && HAVE_DECL___GET_CPUID_MAX
 #include <cpuid.h>
 
 #if defined(__clang__) && defined(__has_attribute)