Provide some vars for use in template's targets.

These new vars are DESTDIR (by default XBPS_DESTDIR/pkgname-version),
SRCPKGDESTDIR (XBPS_DESTDIR/sourcepkg-version) and FILESDIR
(XBPS_TEMPLATESDIR/pkgname/files).

This simplifies packages that used them.

--HG--
extra : convert_revision : 287ea7128cb5df19870ca7aff0a3b8f99a11d63a

templates/gcc/template

@@ -38,7 +38,5 @@ pre_configure()
 
 post_install()
 {
-	local destdir=$XBPS_DESTDIR/$pkgname-$version
-
-	cd $destdir/usr/bin && ln -s gcc cc
+	cd ${DESTDIR}/usr/bin && ln -s gcc cc
 }