Don't compile anything with -msse4.2 unless the compiler supports it.

This includes ws_mempbrk_sse42.c; if the compiler doesn't support
-msse4.2, HAS_SSE4_2 isn't defined, so all the stuff in
ws_mempbrk_sse42.c that uses SSE 4.2 will be #ifdeffed out.

Not all compilers with which we're built will support -msse4.2; in
particular, the ones that aren't compiling for x86 won't....

Change-Id: I69566ca06f602104b40c78b3b06fcb7dfeb054b2
Reviewed-on: https://code.wireshark.org/review/2373
Reviewed-by: Guy Harris <guy@alum.mit.edu>

configure.ac

@@ -981,6 +981,7 @@ then
 		[
 		        have_sse42=yes
 		        AC_DEFINE(HAVE_SSE4_2, 1, [Support SSSE4.2 (Streaming SIMD Extensions 4.2) instructions])
+			CFLAGS_SSE42="-msse4.2"
 		        AC_MSG_RESULT([yes])
 		],
 		[
@@ -996,6 +997,7 @@ else
 fi
 dnl build libwsutil_sse42 only if there is SSE4.2
 AM_CONDITIONAL(SSE42_SUPPORTED, test "x$have_sse42" = "xyes")
+AC_SUBST(CFLAGS_SSE42)
 
 #
 # If we're running GCC or clang define _U_ to be "__attribute__((unused))"

wsutil/Makefile.am

@@ -83,7 +83,7 @@ libwsutil_la_SOURCES =		\
 libwsutil_sse42_la_SOURCES = \
 	ws_mempbrk_sse42.c
 
-libwsutil_sse42_la_CFLAGS = $(AM_CFLAGS) -msse4.2
+libwsutil_sse42_la_CFLAGS = $(AM_CFLAGS) @CFLAGS_SSE42@
 
 EXTRA_libwsutil_la_SOURCES=	\
 	inet_aton.c		\