[Cmake] infinite loop with cmake and mpicxx

You fail to set the compiler for mpi with cmake because you get an infinite loop :

You have changed variables that require your cache to be deleted.
Configure will be re-run and you may have to reset some variables.
The following variables have changed:
CMAKE_CXX_COMPILER= /usr/bin/c++

Well the solution is very simple.

You cannot set the compiler :

SET(CMAKE_CXX_COMPILER mpicxx)

After setting the project :

project(myproject CXX)

So it has to be :

# MPI option has to be set before project
if( USE_MPI )
SET(CMAKE_CXX_COMPILER mpicxx)
endif()

# Project Infos
project(myproject CXX)

#then usual cmakefile

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