T-CI: re-enable macOS build

[sysvinit]

Now that I've (finally) made this build on macOS (at the very least on 10.11, as I don't have any newer machines), we might wish to re-enable the macOS CI builds.

[magnostherobot]

A variety of problems:

Job 8 is failing because gcc-4.9 is not on the path:

make[1]: gcc-4.9: No such file or directory

Job 9 can find gcc-5, so maybe changing gcc-4.9 to gcc-4 would work.

Job 10 and job 11 cannot find an internal header that is used by stdio.h:

In file included from main.c:4:0:
/usr/local/Cellar/gcc@6/6.5.0/lib/gcc/6/gcc/x86_64-apple-darwin17.7.0/6.5.0/include-fixed/stdio.h:78:20: fatal error: _stdio.h: No such file or directory
 #include <_stdio.h>
                    ^

Finally, job 12, the only clang job, cannot find clang at the given path:

make[1]: /usr/local/bin/clang: No such file or directory

[sysvinit]

Hmm, might be worth just disabling the non-functional tests for now so we can at least have some testing on macOS, even if it's only with a handful of compilers.