Use gcc on linux

mentioned in commit 25ee7fb6

merged

Hmm, are there problems with clang on Linux? I occasionally use it to compile kresd, and I haven't encountered any issues yet.

I am trying to isolate Nettle build failure:

checking whether to enable maintainer-specific portions of Makefiles... no
checking ABI=64
checking compiler gcc -O2 -pedantic -fomit-frame-pointer -m64 ... no, mpn_lshift_com optimization 2, program does not run
checking ABI=x32
checking compiler gcc -O2 -pedantic -fomit-frame-pointer -mx32 ... no
checking ABI=32
checking compiler gcc -m32 -O2 -pedantic -fomit-frame-pointer ... no
checking compiler gcc -O2 -pedantic -fomit-frame-pointer ... no, mpn_lshift_com optimization 2, program does not run
configure: error: could not find a working compiler, see config.log for details

I see. In my setup I'm normally doing changes only for knot-dns and knot-resolver packages, not for any other dependencies.

If they only try the gcc command, that won't work. Defining CC=cc might fix such problems generally, assuming travis does always provide cc executable.

I just made a branch that would print last failed config.log (hopefully). I doesn't look like related to used compiler, but something else.