tests: Fix system_elf_gelf_test build without system libelf.h

tests: Fix system_elf_gelf_test build without system libelf.h

If there is no system libelf.h then the building of system-elf-gelf-test
fails with:

../libelf/gelf.h:32:10: fatal error: libelf.h: No such file or directory

This is because although the testcase includes the headers as
../libelf/libelf.h and ../libelf/gelf.h, gelf.h itself does an

Fix this by putting a copy of libelf.h in the build test directory
and using -I. for building system-elf-gelf-test.

	* tests/Makefile.am (BUILT_SOURCES): New for libelf.h.
	(CLEANFILES): Add libelf.h.
	(libelf.h): New target that copies srdir libelf.h.
	(system_elf_gelf_test_CPPFLAGS): Add -I.

https://sourceware.org/bugzilla/show_bug.cgi?id=30812

Signed-off-by: Mark Wielaard <mark@klomp.org>