build: work around ccache/autotools warning-spamming bug

When using clang and ccache, builds spew lots of:
Clang: warning: argument unused during compilation

Upstream bug: https://bugzilla.samba.org/show_bug.cgi?id=8118

This is harmless, bug annoying. If ccache is being used and the
-Qunused-arguments flag is supported (clang), use it.
travis: initial descriptor
travis: add doc
Merge pull request #4749

425c7a8 travis: add doc (Cory Fields)
9380d01 travis: initial descriptor (Cory Fields)
386efb7 build: work around ccache/autotools warning-spamming bug (Cory Fields)