Add PKG_CONFIG_PATH to run.in script.

Allow people to build external bindings using the 'run' script
by defining the PKG_CONFIG_PATH var in it. eg to build Python
you could do

   ../libvirt/run python setup.py build
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

run.in

@@ -53,6 +53,13 @@ else
 fi
 export LD_LIBRARY_PATH
 
+if [ -z "$PKG_CONFIG_PATH" ]; then
+    PKG_CONFIG_PATH="$b/src"
+else
+    PKG_CONFIG_PATH="$b/src:$PKG_CONFIG_PATH"
+fi
+export PKG_CONFIG_PATH
+
 export LIBVIRT_DRIVER_DIR="$b/src/.libs"
 export LIBVIRT_LOCK_MANAGER_PLUGIN_DIR="$b/src/.libs"
 export VIRTLOCKD_PATH="$b/src"