Set linker language to Fortran

CMakeLists.txt

@@ -3,12 +3,18 @@ ecbuild_add_executable (
   SOURCES GEOSgcm.F90
   LIBS GEOSgcm_GridComp ESMF::ESMF OpenMP::OpenMP_Fortran
   )
+# NOTE Due to Gigatraj being a C++ code, CMake apparently
+# gets confused if building with it and thinks GEOSgcm.x
+# is a C++ code and tries to link as C++. This will fail.
+# To fix this, we explicitly set the linker language to Fortran.
+set_target_properties (GEOSgcm.x PROPERTIES LINKER_LANGUAGE Fortran)
 
 ecbuild_add_executable (
   TARGET idfupd.x
   SOURCES idfupd.F90
   LIBS GEOSgcm_GridComp ESMF::ESMF
   )
+set_target_properties (idfupd.x PROPERTIES LINKER_LANGUAGE Fortran)
 
 set (CMAKE_Fortran_FLAGS_RELEASE "${GEOS_Fortran_FLAGS_VECT}")