Fix --enable-libctf and --disable-static

This fixes test runs and compilation when --disable-libctf,
--disable-static, or --enable-shared are passed.

Changes since v2: Use GCC_ENABLE and fix indentation.  Fix prototype
using 'void'.  Use 'unsupported' and gdb_caching_proc.

Changes since v3: Adapt to upstream changes providing skip_ctf_tests.

Changes since v4: Adapt to upstream changes in the seven months (!)
since I last looked at this.

gdb/ChangeLog
	* configure.ac: Add --enable-libctf: handle --disable-static
	properly.
	* acinclude.m4: sinclude ../config/enable.m4.
	* Makefile.in (aclocal_m4_deps): Adjust accordingly.
	(LIBCTF): Substitute in.
	(CTF_DEPS): New, likewise.
	(CLIBS): libctf needs symbols from libbfd: move earlier.
	(CDEPS): Use CTF_DEPS, not LIBCTF, now LIBCTF can include rpath
	flags.
	* ctfread.c: Surround in ENABLE_LIBCTF.
	(elfctf_build_psymtabs) [!ENABLE_LIBCTF]: New stub.
	* configure: Regenerate.
	* config.in: Likewise.

gdb/testsuite/ChangeLog
	* configure.ac: Add --enable-libctf.
	* aclocal.m4: sinclude ../config/enable.m4.
	* Makefile.in (site.exp): Add enable_libctf to site.exp.
	* lib/gdb.exp (skip_ctf_tests): Use it.
	* gdb.base/ctf-constvars.exp: Error message tweak.
	* gdb.base/ctf-ptype.exp: Likewise.
	* configure: Regenerate.