doc: post process html with non-GNU make programs

doc: post process html with non-GNU make programs

* .gitignore (/build-aux/makeinfo-wrapper.sh):
* Makefile.am (EXTRA_DIST): Remove build-aux/makeinfo-wrapper.sh.
* build-aux/makeinfo-wrapper.sh.in: New file, renamed from
build-aux/makeinfo-wrapper.sh and modified to use @MAKEINFO@ instead of
makeinfo.
* cfg.mk (MAKEINFO): Remove variable.
* configure.ac: Add build-aux/makeinfo-wrapper.sh as a configure file.
* doc/local.mk (MAKEINFO): Define to
$(abs_top_builddir)/build-aux/makeinfo-wrapper.sh.
(AM_MAKEINFOFLAGS): Move over some values from the previous MAKEINFO
definition.