Fix gdbsupport build

Fix gdbsupport build

I'm seeing this on F27 (a clean build from scratch):

~~~~~~~~~~~~~~~~~~~~~~~~~~~
 make[3]: Entering directory '/home/pedro/brno/pedro/gdb/binutils-gdb/build/gdbsupport'
   CC       gdb_tilde_expand.o
 In file included from /home/pedro/gdb/binutils-gdb/src/gdbsupport/../gnulib/import/libc-config.h:33:0,
                  from ../gnulib/import/glob.h:544,
                  from /home/pedro/gdb/binutils-gdb/src/gdbsupport/gdb_tilde_expand.c:22:
 ../bfd/config.h:7:4: error: #error config.h must be #included before system headers
  #  error config.h must be #included before system headers
     ^~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~

libc-config.h, where it includes config.h, says:

~~~~~~~~~~~~~~~~~~~~~~~~~~~
 /* This is intended to be a good-enough substitute for glibc system
    macros like those defined in <sys/cdefs.h>, so that Gnulib code
    shared with glibc can do this as the first #include:

      #ifndef _LIBC
      # include <libc-config.h>
      #endif

    When compiled as part of glibc this is a no-op; when compiled as
    part of Gnulib this includes Gnulib's <config.h> and defines macros
    that glibc library code would normally assume.  */

 #include <config.h>
~~~~~~~~~~~~~~~~~~~~~~~~~~~

The issue is that that '#include <config.h>' picks up bfd's config.h
instead of gnulib's.

This problem doesn't trigger in the gdb dir because there we generate
config.h under that exact name so gnulib's libc-config.h ends up
picking gdb's config.h instead of gnulib.c and that ends up harmless.

In gdbsupport, the config.h file is really named support-config.h, so
that '#include <config.h>' in libc-config.h doesn't pick it like it
would if it had the conventional config.h name.

This patch fixes it by simply renaming gdbserver's support-config.h to
config.h.

gdbsupport/ChangeLog:
2020-01-17  Pedro Alves  <palves@redhat.com>

	* configure.ac: Generate config.h instead of support-config.h.
	* common-defs.h: Include <gdbsupport/config.h> instead of
	<gdbsupport/support-config.h>.
	* Makefile.in: Regenerate.
	* configure: Regenerate.