Using $pkgconfig instead of pkg-config will use
${cross_prefix}pkg-config if that is available.

This fix is needed for cross compilations without
modified PATH. Without the fix, PATH must be modified
to find the cross pkg-config before the native
pkg-config.
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>