TRAVIS-CI:  Document how this works

This adds a few words about how we're using travis-CI.
Travis-CI:  Use matrix layout

Because clang doesn't like -W$FOO directives in the CFLAGS, try and generate
jobs which reflect this, by tweaking CFLAGS to just "-g -O2" when clang is
used.
configure.ac:  Appease Clang

For one of the configuration tests, the code-path can have resulted in a
literal "return;" in main() which is invalid because the standard says that
an int should be returned.  Remove this line.

This allows Clang to continue and for ./configure not to fail its tests.