Don't clobber any CXX_FLAGS from the environment.

c++/CMakeLists.txt

-project(Private)
+project("Cap'n Proto" CXX)
 cmake_minimum_required(VERSION 2.8)
 
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 
-set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wno-unused-parameter -std=c++11")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Wno-unused-parameter -std=c++11")
 
 add_subdirectory(src)

c++/src/CMakeLists.txt

-project(Private)
-cmake_minimum_required(VERSION 2.8)
-
-set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wno-unused-parameter -std=c++11")
 
 include_directories(${CMAKE_CURRENT_SOURCE_DIR})