Teach FortranCInterface to verify languages

This module requires both C and Fortran to be enabled, so error-out if
they are not.

Modules/FortranCInterface.cmake

@@ -75,6 +75,15 @@ if(FortranCInterface_SOURCE_DIR)
   return()
 endif()
 
+#-----------------------------------------------------------------------------
+# Verify that C and Fortran are available.
+foreach(lang C Fortran)
+  if(NOT CMAKE_${lang}_COMPILER_LOADED)
+    message(FATAL_ERROR
+      "FortranCInterface requires the ${lang} language to be enabled.")
+  endif()
+endforeach()
+
 #-----------------------------------------------------------------------------
 # Set up an interface detection project.
 set(FortranCInterface_SOURCE_DIR ${CMAKE_ROOT}/Modules/FortranCInterface)