Only enable more warnings in dev mode when requested

Only enable more warnings in dev mode when requested

21ded3ae enabled “more warnings” in dev mode unconditionally, but that
doesn’t play well in dev builds with a Clang version older than the one
used in Travis-CI since the “-Wno-foo” options may be unknown. Instead
of doing feature tests for those options, just enable “more warnings”
when --enable-more-warnings is specified and specify it for Travis-CI
builds.