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

This patch fixes  https://zeromq.jira.com/browse/LIBZMQ-531 (packaging problem detected in Fedora when using the OpenPGM 5.2 system library).

Note that OpenPGM installs a versioned pkgconfig file
(openpgm-5.2.pc, openpgm-5.1.pc).