Set cmake include directories on library targets.

cmake targets that depend on these libraries will automatically have
these include directories.

cmake/libprotobuf-lite.cmake

@@ -24,6 +24,7 @@ set(libprotobuf_lite_files
 
 add_library(libprotobuf-lite ${libprotobuf_lite_files})
 target_link_libraries(libprotobuf-lite ${CMAKE_THREAD_LIBS_INIT})
+target_include_directories(libprotobuf-lite PUBLIC ${protobuf_source_dir}/src)
 set_target_properties(libprotobuf-lite PROPERTIES
     COMPILE_DEFINITIONS LIBPROTOBUF_EXPORTS
     OUTPUT_NAME ${LIB_PREFIX}protobuf-lite)

cmake/libprotobuf.cmake

@@ -54,6 +54,7 @@ set(libprotobuf_files
 
 add_library(libprotobuf ${libprotobuf_lite_files} ${libprotobuf_files})
 target_link_libraries(libprotobuf ${CMAKE_THREAD_LIBS_INIT} ${ZLIB_LIBRARIES})
+target_include_directories(libprotobuf PUBLIC ${protobuf_source_dir}/src)
 set_target_properties(libprotobuf PROPERTIES
     COMPILE_DEFINITIONS LIBPROTOBUF_EXPORTS
     OUTPUT_NAME ${LIB_PREFIX}protobuf)