Check for PKG_CONFIG_LIBDIR when cross compiling.

Check for PKG_CONFIG_LIBDIR when cross compiling.

When cross-compiling, check for PKG_CONFIG_LIBDIR instead of
PKG_CONFIG_PATH. pkg-config searches for pc files in PKG_CONFIG_PATH
*and* the compiled in defaults (/usr/lib/pkgconfig). This means that
pc files from the host get found when cross-compiling.

Setting PKG_CONFIG_LIBDIR makes pkg-config search only in the path
set in PKG_CONFIG_LIBDIR.

Documented in the url below:
http://www.freedesktop.org/wiki/Software/pkg-config/CrossCompileProposal

Change-Id: I22dbf29c5691572b7cb8a5fce712ae7ba811670e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>