Set the -pthread compiler flag when pthreads is used.

Apparently, setting -lpthread is not enough to convince gcc to actually
link a thread library, so if we are using pthreads, set the -pthread
flag as well. It fixes the gcc situation and doesn't have any adverse
effects for clang either.

CMakeLists.txt

@@ -240,7 +240,9 @@ endif (ENABLE_OBJCXX)
 set(CMAKE_THREAD_PREFER_PTHREAD)
 include(FindThreads)
 set(objc_LINK_FLAGS "${objc_LINK_FLAGS} ${CMAKE_THREAD_LIBS_INIT}")
-
+if(CMAKE_USE_PTHREADS_INIT)
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -pthread")
+endif()
 
 
 add_library(objc SHARED ${libobjc_C_SRCS} ${libobjc_ASM_SRCS} ${libobjc_OBJC_SRCS} ${libobjc_CXX_SRCS})