iss: make mfc*.dll and msvc*.dll optional
mfc71.dll was only needed for Python2.4
msvc*.dll is similarly optional
Do not bail installer build if not found
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://mercurial.selenic.com/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.