CMAKE_SOURCE_DIR -> CMAKE_CURRENT_SOURCE_DIR

This tells CMake to refer to the currently executed `CMakeLists.txt`'s directory instead of the root project directory. This allows protobuf-c to be included as a subdirectory (via `include_subdirectory`) while being innocuous for those that build protobuf-c by itself.

build-cmake/CMakeLists.txt

@@ -21,7 +21,7 @@ if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
   SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /wd4267 /wd4244")
 ENDIF()
 
-get_filename_component(MAIN_DIR ${CMAKE_SOURCE_DIR} PATH)
+get_filename_component(MAIN_DIR ${CMAKE_CURRENT_SOURCE_DIR} PATH)
 SET(TEST_DIR ${MAIN_DIR}/t)
 
 MESSAGE(${MAIN_DIR})