PAPI fails to build on Linux systems

[Roland Haas]

When adding support for macOS (non-)builds of PAPI, building on Linux was broken in git hash 7dc06d5 "PAPI: various workarounds for PAPI on macOS M1" of ExternalLibraries-PAPI

Pull request fixes this:

https://github.com/EinsteinToolkit/ExternalLibraries-PAPI/pull/4

[Roland Haas]

Please review.

[Roland Haas]

@Steven R. Brandt says (in pull request) “Looks good to me.”

[Roland Haas]

Applied as git hash 6ee7210 "PAPI: use uname rather than $(TARGET_OS) (#4)" of ExternalLibraries-PAPI

[Roland Haas]

Thank you.