Fix devel/gmp cross-build by telling it about the native C compiler.

Index: devel/gmp/Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/devel/gmp/Makefile,v
retrieving revision 1.68
diff -p -u -r1.68 Makefile
--- devel/gmp/Makefile	12 Feb 2013 09:15:36 -0000	1.68
+++ devel/gmp/Makefile	23 Apr 2013 13:29:15 -0000
@@ -27,6 +27,10 @@ PLIST_VARS+=		cxx
 
 .include "../../mk/bsd.prefs.mk"
 
+.if !empty(USE_CROSS_COMPILE:M[yY][eE][sS])
+CONFIGURE_ENV+=		CC_FOR_BUILD=${NATIVE_CC:Q}
+.endif
+
 .if empty(PKGSRC_COMPILER:Msunpro)
 CONFIGURE_ARGS+=	--enable-cxx
 PLIST.cxx=		yes