Detect Clang masquerading as GCC.

Autoconf detects the Clang compiler as GNU GCC (clang sets defines like
__GNUC__ etc) but Clang is *not* completely compatible. If we detect
Clang we set ac_vc_c_compiler_gnu to 'no'.

configure.ac

@@ -35,6 +35,8 @@ m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
 LT_INIT([win32-dll disable-static pic-only])
 AM_PROG_AS
 AC_PROG_CXX
+XIPH_C_COMPILER_IS_CLANG
+XIPH_GCC_REALLY_IS_GCC
 AC_PROG_MAKE_SET
 
 AC_SYS_LARGEFILE
@@ -521,6 +523,7 @@ AC_MSG_RESULT([
 if test x$ac_cv_c_compiler_gnu = xyes ; then
 	echo "    GCC version : ......................... ${GCC_VERSION}"
 fi
+	echo "    Compiler is Clang : ................... ${xiph_cv_c_compiler_clang}"
 	echo "    Asm optimizations : ................... ${asm_optimisation}"
 	echo "    Ogg/FLAC support : .................... ${have_ogg}"
 echo

m4/clang.m4

+dnl @synopsis XIPH_C_COMPILER_IS_CLANG
+dnl
+dnl Find out if a compiler claiming to be gcc really is gcc (clang lies).
+dnl @version 1.0	Oct 31 2013
+dnl @author Erik de Castro Lopo <erikd AT mega-nerd DOT com>
+dnl
+dnl Permission to use, copy, modify, distribute, and sell this file for any
+dnl purpose is hereby granted without fee, provided that the above copyright
+dnl and this permission notice appear in all copies.  No representations are
+dnl made about the suitability of this software for any purpose.  It is
+dnl provided "as is" without express or implied warranty.
+dnl
+
+
+AC_DEFUN([XIPH_C_COMPILER_IS_CLANG],
+[AC_CACHE_CHECK(whether we are using the CLANG C compiler,
+	xiph_cv_c_compiler_clang,
+	[	AC_LANG_ASSERT(C)
+		AC_TRY_LINK([
+			#include <stdio.h>
+			],
+			[
+			#ifndef __clang__
+				This is not clang!
+			#endif
+			],
+		xiph_cv_c_compiler_clang=yes,
+		xiph_cv_c_compiler_clang=no
+		])
+	)]
+)

m4/really_gcc.m4

+dnl @synopsis XIPH_GCC_REALLY_IS_GCC
+dnl
+dnl Find out if a compiler claiming to be gcc really is gcc (clang lies).
+dnl @version 1.0	Oct 31 2013
+dnl @author Erik de Castro Lopo <erikd AT mega-nerd DOT com>
+dnl
+dnl Permission to use, copy, modify, distribute, and sell this file for any
+dnl purpose is hereby granted without fee, provided that the above copyright
+dnl and this permission notice appear in all copies.  No representations are
+dnl made about the suitability of this software for any purpose.  It is
+dnl provided "as is" without express or implied warranty.
+dnl
+
+# If the configure script has already detected GNU GCC, then make sure it
+# isn't CLANG masquerading as GCC.
+
+AC_DEFUN([XIPH_GCC_REALLY_IS_GCC],
+[	AC_LANG_ASSERT(C)
+	if test "x$ac_cv_c_compiler_gnu" = "xyes" ; then
+		AC_TRY_LINK([
+			#include <stdio.h>
+			],
+			[
+			#ifdef __clang__
+				This is clang!
+			#endif
+			],
+		ac_cv_c_compiler_gnu=yes,
+		ac_cv_c_compiler_gnu=no
+		)
+		fi
+])