Fix FTCBFS

nabi fails to cross build from source, because it looks up pkg-config
using AC_PATH_PROG. A better solution is to use PKG_PROG_PKG_CONFIG, but
since it uses PKG_CHECK_MODULES, this is AC_REQUIREd already. We can
simply drop the AC_PATH_PROG at no loss of functionality and fix the
cross build. Thanks to Helmut Grohne <helmut@subdivi.de>.