force same c++ compiler for unref

modules/slimFunction/CMakeLists.txt

@@ -85,7 +85,7 @@ if(ENABLE_PROJ)
       GIT_REPOSITORY https://git.immc.ucl.ac.be/unref/unref.git
       GIT_TAG a6b4ae1751c44082692ac350040d31907457a669
       INSTALL_COMMAND ""
-      CMAKE_ARGS -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE} -DCMAKE_TOOLCHAIN_FILE=${CMAKE_TOOLCHAIN_FILE} -DEXTERNAL_LIBPROJ=ON -DPROJ_INCLUDE_PATH=${PROJ_INC} -DPROJ_LIB=${PROJ_LIB} -DBUILD_EXECUTABLE=OFF -DCMAKE_CXX_FLAGS=-fPIC
+      CMAKE_ARGS -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE} -DCMAKE_TOOLCHAIN_FILE=${CMAKE_TOOLCHAIN_FILE} -DEXTERNAL_LIBPROJ=ON -DPROJ_INCLUDE_PATH=${PROJ_INC} -DPROJ_LIBRARY=${PROJ_LIB} -DBUILD_EXECUTABLE=OFF -DCMAKE_CXX_FLAGS=-fPIC -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
       PREFIX unref
       BINARY_DIR unref/build
     )