Inherit C++ flags from build environment

As suggested in : http://stackoverflow.com/questions/36765217/how-to-create-cmake-recipes-in-yocto

Build.cmake

@@ -23,7 +23,7 @@ SET(ACTIVE_CPP0xx_DIR "Release")
 IF ("${CMAKE_CXX_COMPILER_ID}" MATCHES ".*Clang")
    MESSAGE("")
    MESSAGE("cmake for Clang ")
-   SET(CMAKE_CXX_FLAGS "-Wall -std=c++11 -Wunused -D_GLIBCXX_USE_NANOSLEEP")
+   SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -std=c++11 -Wunused -D_GLIBCXX_USE_NANOSLEEP")
    IF (${CMAKE_SYSTEM_NAME} STREQUAL "Linux")
        SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libstdc++ -pthread")
    ELSE()
@@ -47,9 +47,9 @@ IF ("${CMAKE_CXX_COMPILER_ID}" MATCHES ".*Clang")
 ELSEIF("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
    MESSAGE("cmake for GCC ")
    IF (APPLE)
-       set(CMAKE_CXX_FLAGS "-Wall -Wunused -std=c++11  -pthread -D_GLIBCXX_USE_NANOSLEEP")
+       set(CMAKE_CXX_FLAGS   "${CMAKE_CXX_FLAGS} -Wall -Wunused -std=c++11  -pthread -D_GLIBCXX_USE_NANOSLEEP")
    ELSEIF (MINGW)
-       set(CMAKE_CXX_FLAGS "-Wall -Wunused -std=c++11  -pthread -D_GLIBCXX_USE_NANOSLEEP -D_GLIBCXX_USE_SCHED_YIELD")
+       set(CMAKE_CXX_FLAGS  "${CMAKE_CXX_FLAGS} -Wall -Wunused -std=c++11  -pthread -D_GLIBCXX_USE_NANOSLEEP -D_GLIBCXX_USE_SCHED_YIELD")
    ELSE()
        set(PLATFORM_LINK_LIBRIES rt)
        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -rdynamic -Wunused -std=c++11 -pthread -lrt -D_GLIBCXX_USE_NANOSLEEP -D_GLIBCXX_USE_SCHED_YIELD")