build: fix mkdtemp check on macOS

On macOS, mkdtemp is in unistd.h. Fix the CMake check so that is works.
This was discovered when comparing the output of the Autotools an CMake
build systems.

CMakeLists.txt

@@ -605,7 +605,7 @@ if(NOT MSVC)
 
   check_cxx_symbol_exists(fork unistd.h HAVE_FORK)
   check_cxx_symbol_exists(gethrtime sys/time.h HAVE_GETHRTIME)
-  check_cxx_symbol_exists(mkdtemp stdlib.h HAVE_MKDTEMP)
+  check_cxx_symbol_exists(mkdtemp "stdlib.h;unistd.h" HAVE_MKDTEMP)
   check_cxx_symbol_exists(accept4 sys/socket.h HAVE_ACCEPT4)
   check_cxx_symbol_exists(strnlen string.h HAVE_STRNLEN)
 else()