Make ./configure --with-system-pgm detect the OpenPGM 5.2 system library.

Make "./configure --with-system-pgm" detect the OpenPGM 5.2 system library.

Note that OpenPGM installs a versioned pkgconfig file (openpgm-5.2.pc,
openpgm-5.1.pc).

PS - This is a backport of https://github.com/zeromq/libzmq/pull/569