Fix `unknown option` warnings when building with `clang`

[StefanJum]

When building musl with clang, a lot of unknown option warnings get generated:

warning: unknown warning option '-Wno-format-contains-nul' [-Wunknown-warning-option]
warning: unknown warning option '-Wno-restrict' [-Wunknown-warning-option]
warning: unknown warning option '-Wno-unused-but-set-variable'; did you mean '-Wno-unused-const-variable'? [-Wunknown-warning-option]
warning: unknown warning option '-Wno-maybe-uninitialized'; did you mean '-Wno-uninitialized'? [-Wunknown-warning-option]

Change the Makefile.uk file to add the flags only if the compiler used is gcc.

Hackathon points: 30

[eduardvintila]

Fixed by #51