Fixing compilation size of shogun binaries in Release/Distribution

I was wondering, why the shogun library is so HUGE even on DCMAKE_BUILD_TYPE=Release or DCMAKE_BUILD_TYPE=Distribution... inspecting the binaries showed, that we're including debug information even for non-debug build.

The fix introduces new variables for debug build flags to make sure, `-g` is not included in Release/Distribution.  In the end, `libshogun.so.16.1` size reduced from 260MB (!) to 23MB.

@vigsterkr, @sonney2k, @besser82 -- It's best if someone of you reviews this.