#745 Fix cmake missing option.

Signed-off-by: Gautier Bureau <gautier.bureau@rte-france.com>

CMakeLists.txt

@@ -13,6 +13,7 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR)
     message(FATAL_ERROR "Do not build in-source. Please remove CMakeCache.txt and the CMakeFiles/ directory. Then build out-of-source.")
 endif()
 
+set(USE_MPI "YES" CACHE STRING "Use MPI")
 set(CMAKE_BUILD_TYPE "Release" CACHE STRING "Choose the type of build, options are: Debug Release RelWithDebInfo MinSizeRel (CMake defaults)")
 
 # Put the project early since modules might need to detect the compiler.