Fix PR1606:

Fix PR1606:
The AC_CHECK_HEADER macro was used instead of AC_CHECK_HEADERS. The former does
not automatically add a #define to the configure variables while the latter
does. Consequently, the HAVE_PTHREAD_H symbol was not defined which caused the
Mutex.cpp file to compile to an empty implementation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@41137 91177308-0d34-0410-b5e6-96231b3b80d8