Since Makefile.maint will soon come from gnulib's maint.mk,
sync Makefile.maint to have the same contents (modulo minor
things).  In syncing it, we have to remove some libvirt-specific
rules.  Since we want to keep them (of course), put those in cfg.mk.
* Makefile.maint: Merge from gnulib's maint.mk.
* cfg.mk (sc_avoid_write): New rule.  From Makefile.cfg.
(sc_prohibit_strcmp_and_strncmp): Likewise, and rename.
(sc_prohibit_asprintf, sc_prohibit_VIR_ERR_NO_MEMORY): Likewise.
(sc_prohibit_nonreentrant): Likewise.
(sc_prohibit_ctype_h): Likewise.
(sc_TAB_in_indentation, sc_avoid_ctype_macros): Likewise.
(sc_prohibit_virBufferAdd_with_string_literal): Likewise.
(sc_prohibit_gethostby): Likewise.
(sc_libvirt_unmarked_diagnostics): Likewise.  Also, rename the
rule, inserting "_libvirt", since this rule is a specialization of
the one in gnulib.
* GNUmakefile: Include cfg.mk, not Makefile.cfg
* .x-sc_prohibit_strcmp_and_strncmp: New file.
* Makefile.am (EXTRA_DIST): Add .x-sc_prohibit_strcmp_and_strncmp