fix macos gcc build by removing unneeded obj-c deps

[d4tocchini]

MacOS build breaks when building with gnu tools via:

CC=gcc-8 CXX=g++-8 cmake ..

Apple builds currently require clang due to Objective-C deps. Turns out gcc support for modern obj-c 2.0 is not trivial without a rabbit hole of GNUstep and what not. Apple specific use of obj-c was isolated to sx_os_numcores and a bit of NSLog, both of which can be replaced with simpler solutions without such heavy handed os deps.

This pull request allows mac builds with gcc, opening potential use with such things as gcc LTO. clang still works, only diff being lighter-weight binaries. I have not tested impact with iOS targets.

[septag]

thanks.
but could you cleanup some more before merging ?

• the changes in jobs.h and job.c should be discarded (formatting changes)
• remove more comments in test-jobs.c
• remove comments from sx.c
• remove comments from CMakeLists.txt

[d4tocchini]

@septag, done & done