What is the difference between AM_CFLAGS, CFLAGS, and
mumble_CFLAGS?

Why does automake output CPPFLAGS after
AM_CPPFLAGS on compile lines?  Shouldn't it be the converse?

My ‘configure’ adds some warning flags into CXXFLAGS.  In
one ‘Makefile.am’ I would like to append a new flag, however if I
put the flag into AM_CXXFLAGS it is prepended to the other
flags, not appended.