gnubg: defer setting PYTHON_CPPFLAGS until pre_configure

The package pulls itself as a hostmakedepends on cross; exporting
PYTHON_CPPFLAGS at the top level of the template will corrupt the build
environment when the native host package must also be built.

srcpkgs/gnubg/template

@@ -18,7 +18,6 @@ LDLIBS="-lm -lreadline -lpython${py3_ver}"
 
 if [ "$CROSS_BUILD" ]; then
 	hostmakedepends+=" ${pkgname}"
-	export PYTHON_CPPFLAGS="-I$XBPS_CROSS_BASE/usr/include/python${py3_ver}"
 fi
 
 case "$XBPS_TARGET_MACHINE" in
@@ -31,5 +30,7 @@ pre_configure() {
 		makeweights < gnubg.weights > gnubg.wd
 		makebearoff -o 6 -s 7999999 -f gnubg_os0.bd
 		makebearoff -t 6x6 -f gnubg_ts0.bd
+
+		export PYTHON_CPPFLAGS="-I${XBPS_CROSS_BASE}/usr/include/python${py3_ver}"
 	fi
 }