Add test-cases b2test_buildid and b3test_dwz_buildid.

The last one triggers the segfault fixed by "[backtrace] Avoid segfault"
( r268275 ).

2019-01-29  Tom de Vries  <tdevries@suse.de>

	* install-debuginfo-for-buildid.sh.in: New script.
	* Makefile.am (check_PROGRAMS): Add b2test and b3test.
	(TESTS): Add b2test_buildid and b3test_dwz_buildid.
	* Makefile.in: Regenerate.
	* configure.ac (HAVE_ELF): Set with AM_CONDITIONAL.
	(READELF): Set with AC_CHECK_PROG.
	(install-debuginfo-for-buildid.sh): Generate with AC_CONFIG_FILES.
	* configure: Regenerate.
	* elf.c (SYSTEM_BUILD_ID_DIR): Factor out of ...
	(elf_open_debugfile_by_buildid): ... here.

From-SVN: r268369