Fix automake forward-incompatibility.
* Added VORBS check to autotools.
* Added -lm on linux as it seems necessary on arch.
* SDL2 should definately not be the default as it's pretty broken still it seems.
Check for sqrt(3), required for system csparse.
Use pkg-config to check for OpenGL and SDL_mixer.
Use default "not-found" behavior for pkg-config.

By default, pkg-config will fail with a detailed error message if a
package is not found; we do not need to specify our own handler.
* Don't actually need glu when compiling.
Remove compiler flags for unnecessary GLU.
Skip check for csparse headers if library not found.

Previous behavior was to check for the system csparse header files
regardless of whether the library was found or not. This led to a linker
failure when a system header was present, even when `configure` tried to
use the internal csparse library.
Bring back original autoconf check for OpenGL.
Merge branch 'master' of https://github.com/bobbens/naev.git
* Fixed invalid outfit.
Merge remote-tracking branch 'upstream/master'