Remove gdb/testsuite/configure

I didn't see a strong reason to have a separate configure script in
gdb/testsuite, so this patch removes it.  The few relevant configury
bits are moved into gdb's configure script.  Some of the old
testsuite/configure script (e.g., the header check) is dead code.

This also adds a Makefile rule to rebuild lib/pdtrace.  This was
missing from the old code.

'read1' is now a dependency of check-read1, rather than extra code at
configure time.

Finally, the old "ENABLE_LIBCTF" subst in gdb/configure was not used;
nor was the variable defined, so this was always empty.  However, the
lower-case variant was used by the testsuite, so this patch renames
the subst.

gdb/ChangeLog
2021-06-01  Tom Tromey  <tromey@adacore.com>

	* configure.ac: Copy some code from testsuite/configure.ac.
	(enable_libctf): Subst this, not ENABLE_LIBCTF.
	* configure: Rebuild.

gdb/testsuite/ChangeLog
2021-06-01  Tom Tromey  <tromey@adacore.com>

	* aclocal.m4, configure.ac, configure: Remove.
	* Makefile.in (EXTRA_RULES): Remove.
	($(abs_builddir)/site.exp site.exp): Don't depend on
	config.status.
	(distclean maintainer-clean realclean, Makefile): Update.
	(config.status): Remove target.
	(lib/pdtrace): New target.
	(all): Don't depend on EXTRA_RULES.
	(check-read1): Depend on read1.so, expect-read1.