Fix CMake build on FreeBSD

FreeBSD's libc has a stub implementation of pthread_once() that returns
ENOSYS and doesn't seem to call the init routine. You need to link with
pthread for this to work.  This PR does this and fixes regress failures
for CMake builds on this platform.  This likely never worked.

CMakeLists.txt

@@ -89,6 +89,10 @@ if(CMAKE_SYSTEM_NAME MATCHES "Linux")
 	set(PLATFORM_LIBS ${PLATFORM_LIBS} pthread)
 endif()
 
+if(CMAKE_SYSTEM_NAME MATCHES "FreeBSD")
+	set(PLATFORM_LIBS ${PLATFORM_LIBS} pthread)
+endif()
+
 if(WIN32)
 	set(BUILD_NC false)
 	if(MINGW)