makedep: Include C derived object files in list of "externals"

Applying makedep to a single shot build of FMS+MOM6 (i.e. skipping the libFMS.a) I found the list of objects was incomplete because the C files were not covered by the module dependencies. makedep figures out the Fortran module dependencies and then has a list of "externals" to cover all the files that don't provide modules. "externals" are always compiled, in lieu of flinting the source code. This commit simply adds all the C object files to the list of externals. When building a library, the module dependencies are ignore and everything gets built, which is why this gap was not seen before.

ac/makedep

@@ -92,6 +92,7 @@ def create_deps(src_dirs, makefile, debug, exec_target, fc_rule,
         _, _, cpp, inc, _ = scan_fortran_file(f)
         # maps object file to .h files included
         o2h[object_file(f)] = cpp
+        externals.append(object_file(f))
 
     # Are we building a library, single or multiple executables?
     targ_libs = []