Fix Mac CI

[macumber]

Fixes #689

@jmarrec I think the better approach would be to upload the new gmp recipe to NREL but I don't have permissions to do that. What do you think we should do?

[jmarrec]

$ conan search -r nrel gmp/6.2.1@ -rev
Revisions for 'gmp/6.2.1' at remote 'nrel':
2011237c81178d014a4c08ae40cfe0cb (2022-06-13 23:14:42 UTC)

This is the PR that fixed it: conan-io/conan-center-index#18447 so it's the 2023-07-10 revision.

$ conan search -r cci gmp/6.2.1@ -rev
Revisions for 'gmp/6.2.1' at remote 'cci':
560eab30a8f52beae2cef5ff26cdd61c (2023-10-25 09:20:17 UTC)
42ae2e6d353a772097058577e0e47004 (2023-08-23 08:16:15 UTC)
83f403337b343ce4530d1e9c5fb3dd68 (2023-07-10 17:27:47 UTC)
c0aec48648a7dff99f293870b95cad36 (2023-03-15 16:26:25 UTC)
9cd6e4a851f6ffd6ea5b5d563f23abcc (2023-01-21 03:36:22 UTC)
83b8f7a110976a83382f74bab17125fd (2022-12-31 10:46:48 UTC)
2011237c81178d014a4c08ae40cfe0cb (2022-02-25 08:13:28 UTC)

From what I can tell, it's no issue to bump to the latest revision. But I'll have to rebuild and host some binaries manually (for eg CCI doesn't have Visual Studio version 17 binaries)

[jmarrec]

I suggest for now we make it global (same gmp on all platforms) with a TODO to remove the pinning once this is no longer needed

[jmarrec]

Haha, of course windows refuses to build that gmp version. I'm tired of this stuff!

[jmarrec]

Thanks @macumber

[jmarrec]

Side note, but setting the variable to empty is unnecessary.

if(NOT WIN32)
  set(CONAN_GMP "gmp/....")
endif()