In the Linux kernel, the BUG() macro causes an instant panic.  Our
BUG() macro is different, however: it generates a nonfatal assertion
failure, and is usable as an expression.

Additionally, this patch tells util_bug.h to make all assertion
failures into fatal conditions when we're building with a static
analysis tool, so that the analysis tool can look for instances
where they're reachable.

Fixes bug 23030.

Wow, it sure seems like some compilers can't implement isnan() and
friends in a way that pleases themselves!

Fixes bug 22915. Bug trigged by 0.2.8.1-alpha and later; caused by
clang 4.

(cherry picked from commit c91a57ccf90308c6728184b43519f96b61acb95d)

(cherry picked from commit 1bb00fb812c0df7a574ed62e9f53b0e8192c7d04)

(cherry picked from commit 8f8689f70235dc19cbc5092ea148af5772a9cdc3)

 * CHANGE .travis.yml so that commands for different purposes (e.g. getting
   dependencies, building, testing) are in separate config lines and sections.
 * CHANGE .travis.yml to use their mechanism for installing dependencies via
   apt. [0]  This also allows us to not need sudo (the "sudo: false" line).
 * CHANGE Travis CI tests (the "script:" section) to build and run tests in the
   same manner as Jenkins (i.e. with --enable-fatal-warnings and
   --disable-silent-rules and run `make check`).
 * ADD Travis configuration to do all the target builds with both GCC and clang.
 * ADD make flags to build with both of the cores available.
 * ADD notifications for IRC, and configure email notifications (to the author
   of the commit) only if the branch was previously building successfully and
   the latest commit broke it.
 * ADD the ability to run the Travis build matrix for OSX as well, but leave it
   commented out by default (because it takes roughly ten times longer, due to a
   shortage of OSX build machines).
 * ADD Travis config option to cancel/fail the build early if one target has
   already failed ("fast_finish: true").
 * ADD comments to describe what our Travis config is doing and why it is
   configured that way.

[0]: https://docs.travis-ci.com/user/installing-dependencies/#Installing-Packages-on-Container-Based-Infrastructure)