fix PAPI 6's build system to build on OSX

[Roland Haas]

PAPI’s support for OSX is not so great and version 6.0 does not compile on OSX due to using some Linux specific functions.

Pull request:

https://github.com/EinsteinToolkit/ExternalLibraries-PAPI/pull/2/files

fixes these issues.

[Roland Haas]

Please review.

[Roland Haas]

@Maria reports (private conversation) that this fixes the compile issues reported in http://lists.einsteintoolkit.org/pipermail/users/2020-September/007611.html

[Roland Haas]

Applied as git hash 5442ec8 "PAPI: add CLFLAGS to LDFLAGS for PAPI's link stage" of ExternalLibraries-PAPI