Add pkgconfig support to PPP project (#270)

Add pkgconfig support to PPP project (#270)

This adds pppd.pc into $(INSTROOT)/$(LIBDIR)/pkgconfig. On some distributions
this would be /lib/pkgconfig, or /usr/lib/pkgconfig, but other distributions
may consider specifying --libdir=/usr/lib/x86_x64-linux-gnu/ and the pkgconfig
directory would be under that. Allowing --libdir to be specified at configure
time fixes #223, providing pkgconfig support fixes #19 and allows third party
packages pickup the plugin directory.

Manually cherry picking parts of two commits by @lkundrak from:
    https://github.com/NetworkManager/ppp/tree/lr/pkgconfig

Mainly, the difference between the original commit is not to replace DESTDIR
with "prefix". Leave DESTDIR alone, and add needed pkgconfig (pppd.pc.in)
as a part of the linux distribution (previously in pppd/plugin directory).

This fixes GitHub issues: #19 #47 and #223

Signed-off-by: Eivind Naess <eivnaes@yahoo.com>