If pdflatex runs out of memory the build fails with no hint what's
wrong. This adds another grep command to the makefile so that an
out-of-memory error will result in more information being shown.

As suggested in https://bugzilla.redhat.com/show_bug.cgi?id=1841056
using lualatex can be used as a workaround.

libstdc++-v3/ChangeLog:

	* doc/Makefile.am (stamp-pdf-doxygen): Also grep for
	out-of-memory error in log file.
	* doc/Makefile.in: Regenerate.

Tested x86_64-linux. Committed to trunk.