Fix x11/libXt cross-build.

The configure script, not just the makefile, needs to know about the
native C compiler.

Index: x11/libXt/Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/x11/libXt/Makefile,v
retrieving revision 1.17
diff -p -u -r1.17 Makefile
--- x11/libXt/Makefile	29 Oct 2012 05:06:36 -0000	1.17
+++ x11/libXt/Makefile	23 Apr 2013 13:29:24 -0000
@@ -28,7 +28,7 @@ CONFIGURE_ARGS+=	--disable-malloc0return
 .endif
 
 .if !empty(USE_CROSS_COMPILE:M[yY][eE][sS])
-MAKE_ENV+=	CC_FOR_BUILD=${NATIVE_CC:Q} CPPFLAGS_FOR_BUILD=-I${PREFIX}/include
+CONFIGURE_ENV+=	CC_FOR_BUILD=${NATIVE_CC:Q} CPPFLAGS_FOR_BUILD=-I${PREFIX}/include
 .endif
 
 CPPFLAGS.OSF1+=		-pthread # Required by compiler if including pthread.h