C, as a cross-platform language is fine...but, make on different platforms gets really ugly. It's not terrible across the modern Open Source platforms, like Linux and FreeBSD (and there's pretty good doc coverage of building compatible makefiles or using GNU autotools), but on systems that don't have the GNU autotools, and have a proprietary make, it can be incredibly painful to deal with.

It's been a few years since I dealt with distributing C across many platforms, but I can't imagine it's changed dramatically for the better (since make has been famously painful for about as long as it has existed).

C's not a bad choice but it's worth mentioning that the last time I installed Ubuntu (admittedly it's been a while) there where three versions of GCC installed automatically, but none of the damn library files (wtf is up with that anyway?). Not a major deal to go fetch the library files but even so...

gcc isn't included with default Mac OS X setup. You need the developer extras in the second DVD.

I believe it's the same for some linux/BSD flavors and installation options.