Fixup for finding pthread.h in CMake

[albinahlback]

@tobiasdiez please check if this works.

[tobiasdiez]

Sadly not:

CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
| Please set them or make sure they are set and tested correctly in the CMake files:
| D:/Programming/sage/subprojects/flint/PThreads_INCLUDE_DIRS
| used as include directory in directory D:/Programming/sage/subprojects/flint
| PThreads_LIBRARIES (ADVANCED)


 CMake Error in CMakeLists.txt:
| Target "flint" INTERFACE_INCLUDE_DIRECTORIES property contains path:

| "D:/Programming/sage/subprojects/flint/PThreads_INCLUDE_DIRS-NOTFOUND"

btw, is pthreads completely optional?

[albinahlback]

Yes, pthreads is optional, but I would like to avoid too many branches in CMake, so I would like to keep it mandatory in CMake. How do you proceed for other projects with your devel-Windows-thing for pthreads?

[tobiasdiez]

I don't know - it's my first time as well and I'm honestly a bit confused by all the different pthreads ;-) So I would welcome if it could be made completely optional also for cmake.

[tobiasdiez]

What works for me is

find_path(PThreads_INCLUDE_DIRS NAMES pthread.h )
find_library(PThreads_LIBRARIES NAMES libwinpthread.dll pthreads libpthreads pthread)

instead of line 138 to 144 (i.e. without the HAVE_PTHREAD_H check, which I think is made obsolete by the find_path).

[albinahlback]

Please make a code suggestion.

[isuruf]

See #2171

[albinahlback]

Closing in favor of #2171.