configure.ac pkg-config macro not cross-compilation safe

The debian package checking tool 'lintian' spotted that the macro used to detect pkg-config existance is not cross-compilation safe.

Full description at: https://lintian.debian.org/tags/autotools-pkg-config-macro-not-cross-compilation-safe.html

Basically replace: AC_PATH_PROG(PKG_CONFIG, pkg-config, no) with: PKG_PROG_PKG_CONFIG in configure.ac to fix this (and possibly also include pkg.m4 in m4/).