libbacktrace: avoid infinite recursion

We could get an infinite recursion in an odd case in which a
.gnu_debugdata section was added to a debug file, and mini_debuginfo
was put into the debug file, and the debug file was put into a
/usr/lib/debug directory to be found by build ID.  This combination
doesn't really make sense but we shouldn't get an infinite recursion.

	* elf.c (elf_add): Don't use .gnu_debugdata if we are already
	reading a debuginfo file.
	* Makefile.am (m2test_*): New test targets.
	(CHECK_PROGRAMS): Add m2test.
	(MAKETESTS): Add m2test_minidebug2.
	(%_minidebug2): New pattern.
	(CLEANFILES): Remove minidebug2 files.
	* Makefile.in: Regenerate.