Is DOSBox a reliable environment to compile DOS sources or would you suggest a different emulator/environment?

Hmm, in the past when that came up or when the question was about math programs that needed to be trustworthy, I got the impression that you shouldn't use Dosbox. Some needed stuff might be missing or so. But I could be wrong and the last time that came up was really some time ago.
I'm not sure if any emulator should be used. Maybe bochs.

If you've ever looked at the sources of some emulator/virtual machine,
you'll never trust it for anything (that includes commercial solutions imo).
But of course a real system isn't fail-proof, bad ram may have the most
subtle effects.

To compile or play a *game* you can use anything that works, the errors
you describe are most likely missing headers/libraries.

You could probably make a djgpp cross compiler and compile on windows/linux and run the created executables in dosbox. The open watcom compiler can also produce dos binaries without actually being run under dos i think.

Congratulations and welcome to the world of programming, or at least the section of sidewalk in front of the house where programming lives. If you're going to be compiling BASIC, I recommend FreeBASIC, it works well, has a nice front-end, and can compile DOS executables without being in DOS, as well as some other systems, and even the Xbox if you have or get one and decide to install Linux on it. If it's C, I'd probably say GCC, but I'm not nearly an expert on that. If it's Pascal, the above mentioned OpenWatcom seems to be a good choice, but I'm even less sure about that (edit: and apparently wrong). Good luck.

Thanks for the advice! Things are getting better. I wiped the whole DJPP directory and re-did it from start.
I guess I'm picking the wrong examples to compile for a newbie (let's say a few thingies with lot of dependencies) and I'm getting lots of "deprecated" errors. I'm also making some confusion about different languages but I guess something.c should be compiled with the gcc thingie and something.cc with gpp, correct?
Also most difficult thing for me is matching the different versions of DJGPP, Allegro and DUMB so I decided to get the latest versions of each but when I compiled the DUMB libraries with make it said "Nothing to be done for Allegro..."

Deprecation is a warning, not an error. It means that a piece of an API is outdated and may, at some point in the future, be removed entirely. While it's something that should probably be cleaned up, it will not cause compilation to fail.

Just a quick note: OpenWatcom is not a Pascal compiler. There is C/C++ and also Fortran.

i'd stay far, far away from DOSBox or Windows NT when compiling with DJGPP (even when the zip picker has appropriate options for those you'll most likely run into segfaults). I personally find the fastest / most stable way for that is Virtual PC + Windows 98SE.