BUILD: Error out if no working C++ compiler could be found

Because apparently, AC_PROG_CXX doesn't. -.-
Revert "BUILD: Error out if no working C++ compiler could be found"

This reverts commit 144b443b1fd7b873655b5d47ee40e3f3e151ef0d.

On second examination, this is unfortunately not a good idea.
AC_CHECK_PROG doesn't work with absolute paths, so this now
breaks the usecase of someone having set CXX to an absolute path.

Moreover, from a cursory grep over various source distributions,
I found that no one else checks for CXX existing either.
BUILD: Add Travis CI support.