Restore 'coverage' Makefile target.

Ancillary changes to configure.ac/Makefile.am to ensure that both
CFLAGS and AM_CFLAGS are honored for all object files and don't
step on each other.

(AM_CFLAGS is only applied to targets that don't have a <target>_CFLAGS of their own, so we either have to not define <target>_CFLAGS, or we have to explicitly add AM_CFLAGS to <target>_CFLAGS.)