Add a useful kludge to allow package maintainers to specify compiler and other

Add a useful kludge to allow package maintainers to specify compiler and other
platforms details on the command line without having to patch the Configure
script everytime: One now can use ``perl Configure <id>:<details>'', i.e.
platform ids are allowed to have details appended to them (seperated by
colons). This is treated as there would be a static pre-configured entry in
Configure's %table under key <id> with value <details> and ``perl Configure
<id>'' is called.  So, when you want to perform a quick test-compile under
FreeBSD 3.1 with pgcc and without assembler stuff you can use ``perl Configure
"FreeBSD-elf:pgcc:-O6:::"'' now, which overrides the FreeBSD-elf entry
on-the-fly.

(PS: Notice that the same effect _cannot_ be achieved by using
     ``make CC=pgcc ..'' etc, because you cannot override all
     things from there.)