Support for ParMmg as a subproject and modern cmake

[prudhomm]

ParMmg cannot be used as a subproject. A few changes in the CMakeLists.txt would allow that. For example use ${PROJECT_BINARY_DIR} instead of ${CMAKE_BINARY_DIR}. Also target_include_directories could be improved and targets export could be setup

[lcirrottola]

Thank you for the feedback and the improvements!