Merge pull request #2343 from philip-wernersbach/fix-pthreads-under-cpp

Fix issues with pthread_create when compiling under C++ mode.

tests/cpp/tthread_createthread.nim

@@ -0,0 +1,14 @@
+discard """
+  cmd: "nim cpp --hints:on --threads:on $options $file"
+"""
+
+proc threadMain(a: int) {.thread.} =
+    discard
+
+proc main() =
+    var thread: TThread[int]
+
+    thread.createThread(threadMain, 0)
+    thread.joinThreads()
+
+main()