configure questions

I am currently working on bug 134113.
The first step is to add a special switch (that will be set in mozconfig 
or by passing to configure) that will tell the configure system that you 
want to use GCC instead of MSVC to build with.
Questions:
1.what should I use as a switch? (I recon something like use_cygwin_gcc 
but I want to make sure I folow any established standards for this)
2.how would this then get set in the mozconfig? (would I just add 
use_cygwin_gcc=1 to mozconfig)?
3.is there anything special I need to do to get my new flag recognized 
by the build system (some master list of flags or something?)
4.how do I make sure my flag is passed onto the configure scripts for NSPR?
and 5.within the configure script, how do I test the status of my flag? 
(looking at existing examples, its got something to do with a command 
called test, how do I use that?)

Basicly, what I plan to do is to take this code:
esac

if test -n "$_WIN32_MSVC"; then
and insert between the esac and the if, some code that will do something 
like this
if use_cygwin_gcc then
unset _win32_msvc
end if

Either that or change the test for _win32_msvc so that it tests for 
_win32_msvc as before but also tests to see that use_cygwin_gcc is clear 
before setting the skip flags.

Then, set the paths so that cygwin is before msvc and build, letting the 
configure script figure out that its using GCC and what flags that means 
it needs to use. Then we can see where it gives errors :)