mesa: Add -fno-math-errno to CFLAGS.

Cuts about 9k of .text size.

   text    data     bss     dec     hex filename
4992804  197808   26328 5216940  4f9aac i965_dri.so before
4983676  197808   26328 5207812  4f7704 i965_dri.so after

Also, Darwin's libm does not ever set errno, so if we care about those
systems we shouldn't rely on errno anyway.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>

configure.ac

@@ -286,6 +286,9 @@ if test "x$GCC" = xyes; then
     # Work around aliasing bugs - developers should comment this out
     CFLAGS="$CFLAGS -fno-strict-aliasing"
 
+    # We don't want floating-point math functions to set errno
+    CFLAGS="$CFLAGS -fno-math-errno"
+
     # gcc's builtin memcmp is slower than glibc's
     # http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43052
     CFLAGS="$CFLAGS -fno-builtin-memcmp"