In cflags.SH trim the case statement of file basenames.

In cflags.SH trim the case statement of file basenames.

Add examples showing how to use cflags.SH to tweak the compiler flags used
for individual object files.

Previously cflags.SH contained a somewhat stale pre-canned list of file
basenames including removed files such as usersub (deleted before 5.000
shipped), and a partial list of 5.000 XS extensions. Whilst it's possible
to generate the correct list in cflags by parsing MANIFEST (and adding a few
fixups), it's still not actually *useful*, as cflags gets overwritten as
soon as config.sh changes. Hence the most end-user useful solution with
minimal maintenance is to eliminate the list entirely, and document how the
user should add to it as necessary.