Add support for VPATH builds, that is, building somewhere else than in the

source directory.  This involves mostly makefiles using $(srcdir) when they
might have used ".".  (Regression tests don't work with this, yet.)

Sort out usage of CPPFLAGS, CFLAGS (and CXXFLAGS).  Add "override" keyword
in most places, to preserve necessary flags even when the user overrode the
flags.