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 #569 from jpoliv/master

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