mesa: added #ifdef __GNUC__ around GLubyte bitfield usage

It would be nice if there were a #pragma or something to disable the warnings:

main/texenvprogram.c:87: warning: type of bit-field ‘Source’ is a GCC extension
main/texenvprogram.c:88: warning: type of bit-field ‘Operand’ is a GCC extension

but there doesn't appear to be a way to do that.

src/mesa/main/texenvprogram.c

@@ -82,8 +82,13 @@ texenv_doing_secondary_color(GLcontext *ctx)
 #define DISASSEM (MESA_VERBOSE & VERBOSE_DISASSEM)
 
 struct mode_opt {
+#ifdef __GNUC__
    GLubyte Source:4;  /**< SRC_x */
    GLubyte Operand:3; /**< OPR_x */
+#else
+   GLubyte Source;  /**< SRC_x */
+   GLubyte Operand; /**< OPR_x */
+#endif
 };
 
 struct state_key {