Fix bug that flowgrindd -d doesn't work if configured w/o --enable-debug
[Coding style] add missing comments to preprocessor commands
Fix warning while building on OS X
Fix warning while building on OS X

Helper function rn_uniform, rn_uniform_zero_to_one, and
rn_uniform_minusone_to_one are only used if libgsl is not available.
[Coding style] break long lines
[Coding style] delete whitespaces
Use errx instead of warnx for sanity checks
Fix doxygen warnings