Draft configure support

Draft configure support

I became to conclusion that using autoconf alone, without aclocal,
automake and libtool, is not that bad - together with relying on
pkg-config, workable configure could be created.

To not depend on aclocal, we'll ship pkg.m4 in our srctree. So far this
is the only addiotional-to-what autoconf already provides out of the box
module, and I expect it to 99% stay that way.

This patch adds configure stub which only finds pkg-config, plus
Makefile hooks, to reconfigure/regenerate-configure/etc. The first
configure user will come in the next patch.

pkg.m4 taken from  pkg-config.git pkg-config-0.26-2-gb93f795

Signed-off-by: Kirill Smelkov <kirr@navytux.spb.ru>