libstdc++: Allow lualatex to be used for Doxygen PDF

This allows the Doxygen PDF to be built using lualatex instead of
pdflatex, which solves a problem with pdflatex running out of memory
sometimes. This is done by adding a --latex_cmd option to the
run_doxygen script, which then sets the specified command in the
generated user.cfg file used by Doxygen. The makefile is adjusted to
pass --latex_cmd=$(LATEX_CMD) to the script, so using running make with
LATEX_CMD=lualatex will override the default.

Additionally, this does some refactoring of the doc/Makefile.am rules
and the run_doxygen script.

libstdc++-v3/ChangeLog:

	* doc/Makefile.am: Simplify doxygen recipes and use --latex_cmd.
	* doc/Makefile.in: Regenerate.
	* doc/doxygen/user.cfg.in (LATEX_CMD_NAME): Add placeholder
	value.
	* scripts/run_doxygen (print_usage): Always print to stdout and
	do not exit.
	(fail): New function for exiting on error.
	(parse_options): Handle --latex_cmd. Do not treat --help the
	same as errors. Simplify handling of required arguments.