We have two mappings for the "/MAP" flag.  The first does not care
whether there is a value and activates the GenerateMapFile boolean
setting.  The second takes a value and puts it in the MapFileName
setting.  The latter must treat the ":" as part of the flag.

This is similar to commit 8ae66bf4 (Fix optionally-valued booleans in VS
10 flag table, 2009-10-23).