Fix OS X invalid renderer bug, update gitignores

This PR updates the extlib .gitignores, to ignore some more built files.

The `SDLOTHERCONFIG` thing basically makes it so that flags like enabling opengl actually take effect, and don't just get squashed by the `--disable-opengl` right after it. This makes OpenGL-based OS X rendering actually work.

The printfs and error messages are just split out a bit more, to more properly see exactly where launching issues are. As it was before , it was pointing me towards an invalid texture error, when what it really meant was an invalid renderer (since opengl was forced to be disabled).

For reference, I've now got this compiling on my local OS X with the following:
```CC="clang" CXX="clang++" SDLOTHERCONFIG="--enable-video-opengl --enable-video-opengles" ./configure --unsupported-compiler --with-internal-libs
CC="clang" CXX="clang++" SDLOTHERCONFIG="--enable-video-opengl --enable-video-opengles" make```

You need to make sure you don't have SDL2 installed via Homebrew however, or else it stuffs the internal libs compiling (Looks like it uses the system sdl2-config instead of internal when HB version's also installed, but I'll need to look into it).