crossroads builds an internal libpgm from source, but that causes
build errors because of a conflicting struct definition with OSX
`netinet.h`. So instead of dealing with that, link crossroads
against our libpgm-5.2 which builds without error. Add a couple
of ENV vars to override `pkg-config` and help crossroads find
the libpgm we now depend on, which it can't find otherwise because
it's looking for `openpgm-5.1.pc` and we offer `openpgm-5.2.pc`.
Crossroads passes `make check` on 10.8.2 with XCode-4.5.
FixesHomebrew/homebrew#15217ClosesHomebrew/homebrew#15255.
Signed-off-by: Adam Vandenberg <flangy@gmail.com>
