#3 General project maintenance & Windows build fix

I hope the commits are fairly self-explanatory. Here I'll just explain some of the reasoning behind the changes:

• [1273ef4] PrBoom+ compile warnings are now disabled, as these aren't really relevant to XDRE and just clutter the compilation output.
• [8e56a55] There must exist a better fix for this issue, but this is what happens to work for me: I was getting make errors due to the rdatawad executable ("No rule to make target 'rdatawad' [...] Stop."). This was fixed by manually appending the 'exe' file extension in the relevant Makefile.am script. Now, Autotools will generate prboom-plus.wad by running rdatawad.exe as a Windows executable through Wine.
  I suspect that the correct solution would be to somehow tell Autotools to build rdatawad as a Linux executable, and then run it natively to generate prboom-plus.wad, thus not requiring Wine.
• [d26e9bb] On my system, the general '-static' linker flag wasn't enough to statically link SDL2. The build would always end in linker errors. This was because sdl2.m4 was missing the ability to request static linker flags, which I then added myself. This flag now used in the Windows build scripts.
• [362866a5ea] The reason the XDRE 2.21 release "didn't work" on Windows was only because of the absence of the prboom-plus.wad file. The program actually runs perfectly fine, but exits early when it can't find this file. The program even prints a warning message to the user to stdout, but because Windows GUI programs don't print stdout to the command-line like on Linux, it just looks like the program won't load.

Closing in favor of earlier pull request #2. Both pull request are now the same.