package/Makefile.in: Use appropriate paths for host pkg-config.

package/Makefile.in: Use appropriate paths for host pkg-config.

PKG_CONFIG_PATH adds a path to the pkg-config search path, but this is not
enough when we are building host tools - the host pkg-config default path
is still used, and this will pick up .pc files in the staging_dir tree.

PKG_CONFIG_LIBDIR overrides the pkg-config default path and ensures only
host .pc files are found. This fixes a pango build failure when building
for the host with cairo PNG support enabled.

Signed-off-by: Will Newton <will.newton@gmail.com>