[CMake] CHECK_INCLUDE_FILE_CXX with built from source compiler

[houssen]

Hello,

I do NOT use /usr/bin/g++ that is installed in the system (long story - 
don't ask why).
I compiled gcc from source and installed it in /my/path/local/bin/g++.

Now I need to check for tuple with CHECK_INCLUDE_FILE_CXX : it fails !

Still failing if:
1. I export CPLUS_INCLUDE_PATH to point at 
/my/path/local/include/c++/x.y.z (I checked: tuple is here)
2. set this before calling CHECK_INCLUDE_FILE_CXX :
   SET ( CMAKE_REQUIRED_FLAGS "-I$ENV{CPLUS_INCLUDE_PATH}" )
   SET ( CMAKE_REQUIRED_INCLUDES "$ENV{CPLUS_INCLUDE_PATH}" )

What is wrong ?

Franck