Check for pthread.h in CMake if no package could be found

When having `winpthreads-devel` from conda installed,
`find_package(PThreads)` does not succeed although `pthread.h` is
present.

Co-authored-by: Tobias Diez <[email protected]>

CMakeLists.txt

@@ -133,7 +133,14 @@ if(MSVC)
         set(PThreads_LIBRARIES PThreads4W::PThreads4W)
         set(PThreads_INCLUDE_DIRS ${PThreads4W_INCLUDE_DIRS})
     else()
-        find_package(PThreads REQUIRED)
+        find_package(PThreads)
+        if(NOT PThreads_FOUND)
+            include(CheckIncludeFile)
+            check_include_file(pthread.h HAVE_PTHREAD_H)
+            if(NOT HAVE_PTHREAD_H)
+                message(FATAL_ERROR "Could not find pthreads.")
+            endif()
+        endif()
     endif()
     set(FLINT_USES_PTHREAD ON CACHE BOOL "Use POSIX Threads.")
 else()