cmake: include threading libraries in pkg-config

Include any required threading libraries in our `libgit2.pc`.

Author: Edward Thomson

CMakeLists.txt

@@ -161,6 +161,8 @@ FUNCTION(TARGET_OS_LIBRARIES target)
 
 	IF(THREADSAFE)
 		TARGET_LINK_LIBRARIES(${target} ${CMAKE_THREAD_LIBS_INIT})
+		LIST(APPEND LIBGIT2_PC_LIBS ${CMAKE_THREAD_LIBS_INIT})
+		SET(LIBGIT2_PC_LIBS ${LIBGIT2_PC_LIBS} PARENT_SCOPE)
 	ENDIF()
 ENDFUNCTION()