read --build-id from root cmake project during linking (#7)

CMakeLists.txt

@@ -6,7 +6,7 @@ include(common.cmake)
 include(thirdparty.cmake)
 
 # Setup build ID via the linker. Also, statically link GCC libraries to lock GCC version.
-execute_process(COMMAND git rev-parse --sq HEAD OUTPUT_VARIABLE GIT_COMMIT WORKING_DIRECTORY ${PROJECT_SOURCE_DIR})
+execute_process(COMMAND git -C ${CMAKE_SOURCE_DIR} rev-parse --sq HEAD OUTPUT_VARIABLE GIT_COMMIT WORKING_DIRECTORY ${PROJECT_SOURCE_DIR})
 set(CMAKE_EXE_LINKER_FLAGS "-Wl,--build-id=0x${GIT_COMMIT} -static-libstdc++ -static-libgcc ${ENVOY_EXE_EXTRA_LINKER_FLAGS} ${CMAKE_EXE_LINKER_FLAGS}")
 
 include_directories(${PROJECT_SOURCE_DIR}/include)