preprocessor flags (such as -I). CXXFLAGS is for C++ compilation flags
(such as -Wall).

* Set AM_CFLAGS and AM_CXXFLAGS in configure.ac so that each Makefile.am
gets default warning flags. This eliminates the duplication of -Wall in a
dozen places.

* Set the compliation warning flags to -Wall -Wextra -Wpedantic, so that
we see all the warnings. We can't fix warnings we never see.