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).
Merge pull request #107 from jpoliv/master

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