To start with, I decided to try compiling the latest update of the GNU Fortran compiler. (Since I'm asking about the possibility of getting write access to the official source repository to commit updates, I really ought to have a working installation of it, even if I'm only planning to work on documentation....) This is a fairly long and involved process, involving compiling all of GCC, and it takes about three hours or so on my computer. Except that it seems that this computer is prone to very occasional "permission denied" errors on perfectly good files that were accessed only a moment ago, and since the GCC build process is poking a different file every couple of seconds for those three hours ... well, it seems to be pretty much a statistical certainty that somewhere in the process it's going to error out, after an hour or two. And the build process really isn't very fault-tolerant for this.
Eventually, I decided to try updating the Cygwin installation, to see if that happened to fix the problem. And discovered that I'm also having trouble connecting to the Cygwin repositories, so that after I'd spent 15 minutes going through the list of bits and pieces it wanted to update, and pruning the list down to something that was reasonable to download (no, I don't need a 15MB update to a program that I downloaded out of curiousity and haven't ever actually used), it hit a "can't connect to server" error, which cancels the entire download process so that I have to do that all over again.
I really didn't need this today, you know?
Addendum: And, when I tried copmiling GCC on my Debian box, I discovered that even an up-to-date install of Debian Stable doesn't have sufficiently recent math libraries to compile GNU Fortran, so it won't compile there either. On the other hand, I should note that my actual work programming went pretty well today, so perhaps I shouldn't complain too much.