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
Use ap_is_used() instead of of an own local bool variable

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Merge pull request #136 from alexander-zimmermann/next

Fix bug & 2 warnings on OS X