xbps-src: set up C{,XX}FLAGS and LDFLAGS correctly for cross compilation.

This way there's no need to set LDFLAGS per-pkg just to fix cross compilation
in multiple packages.

common/environment/configure/cross.sh

@@ -0,0 +1,5 @@
+if [ -n "$CROSS_BUILD" ]; then
+	CFLAGS+=" -I${XBPS_CROSS_BASE}/usr/include"
+	CXXFLAGS+=" -I${XBPS_CROSS_BASE}/usr/include"
+	LDFLAGS+=" -L${XBPS_CROSS_BASE}/usr/lib"
+fi