vala: prefix important variables in pkg-config file with pc_sysrootdir

To allow cross compilation we need to pass those variables with
the pc_sysrootdir prefix, otherwise the values will be from
the host system.

srcpkgs/vala/template

@@ -1,7 +1,7 @@
 # Template file for 'vala'
 pkgname=vala
 version=0.42.5
-revision=1
+revision=2
 build_style=gnu-configure
 configure_args="--disable-graphviz"
 hostmakedepends="flex libxslt pkg-config automake libtool"
@@ -28,6 +28,18 @@ pre_configure() {
 	autoreconf -fi
 }
 
+post_install() {
+	# Prefix the variables 'datadir' 'vapidir' and 'vapidir_versioned'
+	# with pc_sysrootdir so it gets expanded to the value of
+	# PKG_CONFIG_SYSROOT_DIR, this is necessary for packages that
+	# query pkg-config for the variables as we need to return the
+	# values from the cross environment not the native one.
+	vsed -e 's|^datadir=.*|datadir=${pc_sysrootdir}/${datarootdir}|g' \
+		 -e 's|^vapidir=.*|vapidir=${pc_sysrootdir}/${datadir}/vala/vapi|g' \
+		 -e 's|^vapidir_versioned=.*|vapidir_versioned=${pc_sysrootdir}/${datadir}/vala-0.42/vapi|g' \
+		 -i ${DESTDIR}/usr/lib/pkgconfig/vapigen-0.42.pc
+}
+
 libvala_package() {
 	short_desc+=" - shared library"
 	pkg_install() {