Fix bugs with integer-valued variables when parsing Makefiles. Values

Fix bugs with integer-valued variables when parsing Makefiles.  Values
for done[n] can be integers as well as strings, but the code
concatenates them with strings (fixed by adding a str()) and calls
string.strip() on them (fixed by rearranging the logic)

(Presumably this wasn't noticed previously because parse_makefile()
was only called on Modules/Makefile, which contains no integer-valued
variables.)