-Override the default compiler by specifying a CXX in
Makefile.config instead of CUSTOM_CXX, as Travis exports CXX
as the compiler env variable name.
-Change TEST_HDFS -> TEST_HXX_SRCS.
@jeffdonahue's tweaks to .travis.yml config:
-Add (CPU-only) test, lint, warn and parallel (-j 4) to travis CI build.
-Add /usr/local/lib to travis config (seems needed for LMDB).
-Put export in "before_script"; disable clang build -- doesn't work on Linux.
-Cache Ubuntu apt packages.
-Install bc package to hopefully suppress "bc: not found" errors
-Get apt packages before_install as suggested by Travis official docs
-Remove specified email address and IRC notifications (emails are sent
to the committer by default; others can view build results in public
Travis feed, on pull requests, etc.).
Travis build failure wasn't working for lint/warn because they didn't
exit with a non-zero code -- this fixes that.