Add libxml2 and pkg-config to build dependencies for mpich and set PKG_CONFIG_PATH (fixes issue #701).

Author: johannesring

pkgs/mpich.yaml

@@ -1,4 +1,6 @@
 extends: [autotools_package]
+dependencies:
+  build: [libxml2, pkg-config]
 
 build_stages:
   - name: configure
@@ -27,3 +29,5 @@ when_build_dependency:
 
   - prepend_path: PATH
     value: ${ARTIFACT}/bin
+  - prepend_path: PKG_CONFIG_PATH
+    value: ${ARTIFACT}/lib/pkgconfig