Some shared fixes

I thought the build matrix would look a bit nicer with unicode symbols (✓ and ✗) instead of "Y" and "N".

I got the "Argument list too long" error with the build-matrix.html target, someone with more make-foo then me could split that special "line" into multiple commands. My current "fix" seems to get the line length short enough so it doesn't happen any more (Debian stable).

And some more updates for shared builds. I reused the test program for smpeg in smpeg2 and the test program from sdl_mixer in sdl2_mixer. Is that a viable solution? Otherwise I revise these patches and copy the test program...

eigen was simple since it doesn't build any libraries at all (only header files).