.travis.yml: add cmake support

Simplify PKG_CONFIG_PATH and LDFLAGS while at it using the new
--enable-asan option which automatically adds the required flags if
available. LDFLAGS is ignored by cmake, but apparently cmake builds
packages with the correct rpath without any further options.

Additionally, build examples by default (without -Werror) and remove
"cat config.log" which would result in configure succeeding while it
should not.