Add ISSM ice sheet model GridComp to Landice

GEOSagcm_GridComp/GEOSphysics_GridComp/GEOSsurface_GridComp/GEOSlandice_GridComp/CMakeLists.txt

@@ -1,6 +1,40 @@
 esma_set_this ()
 
+# First set alldirs to empty
+set(alldirs)
+# ... and srcs to the LandIceGridComp
+set (srcs GEOS_LandIceGridComp.F90)
+
+# Then try to find ISSM
+find_package (ISSM QUIET COMPONENTS Core)
+
+if(ISSM_FOUND)
+  # If ISSM is found, append the directory to alldirs
+  message(STATUS "ISSM found, building gridded component")
+  list (APPEND alldirs GEOSissm_GridComp)
+else()
+  # If ISSM is not found, append the stub component to srcs
+  message(STATUS "ISSM not found, using stub component")
+  # For esma_create_stub_component, we need the name of the *module*
+  # that the landice gc is expecting, minus the mod. Also we
+  # have to use the variable srcs but not the ${srcs} because
+  # esma_create_stub_component is expecting the list itself, not the
+  # contents of the list.
+  esma_create_stub_component(srcs GEOS_IssmGridComp)
+endif()
+
+# We have to do the above way because what esma_create_stub_component
+# does is to append the stub component it creates in the build tree to a
+# list of sources srcs.  So if ISSM is found, we go into the subdir and
+# build the real component, and if ISSM is not found, we create the stub
+# component and append it to srcs.
+
 esma_add_library (${this}
-  SRCS GEOS_LandIceGridComp.F90
-  DEPENDENCIES MAPL GEOS_Shared GEOS_SurfaceShared ESMF::ESMF NetCDF::NetCDF_Fortran
-  )
+  SRCS ${srcs}
+  SUBCOMPONENTS ${alldirs}
+  DEPENDENCIES MAPL GEOS_Shared GEOS_SurfaceShared ESMF::ESMF NetCDF::NetCDF_Fortran)
+
+if(ISSM_FOUND)
+  # If ISSM is found, add this definition
+  target_compile_definitions(${this} PRIVATE HAVE_ISSM)
+endif()