This should fix more exotic POSIX platforms such as HPUX which insist on
having _POSIX_C_SOURCE defined to get proper threading behaviour. We also
define _GNU_SOURCE so that we get any extensions on systems with glibc, e.g.
eventfd.

Disallow build --with-pgm on non-x86/AMB64 CPUs
Check correctly for all dependencies required by --with-pgm
Don't assume tar has -j
Reorder --with-pgm checks

As decided on the mailing list, there have been five 2.x releases to
date, so the upcoming release will be numbered 2.0.6.

configure.in is now the master source for the package version number,
this propagates to src/platform.hpp (for zmq_version) and doc/Makefile.am
(for documentation generation) automagically.