gdbserver: modernize configure.ac

gdbserver: modernize configure.ac

Run autoupdate on gdbserver/configure.ac and then tweak it to use easier
to read indentation.  This removes a few warnings when running
`autoreconf -vf -Wall`.

  * Replace AC_INIT with AC_INIT and no arguments plus AC_CONFIG_SRCDIR.
  * Replace AC_GNU_SOURCE with AC_USE_SYSTEM_EXTENSIONS.
  * Replace AC_TRY_COMPILE with AC_COMPILE_IFELSE.
  * Replace AC_TRY_LINK with AC_LINK_IFELSE.

autoupdate gets it right, except this one here:

    --- a/gdbserver/configure.ac
    +++ b/gdbserver/configure.ac
    @@ -304,7 +304,7 @@ if test "$srv_linux_thread_db" = "yes"; then
         AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[]])],[found="-Wl,--dynamic-list"
                     RDYNAMIC='-Wl,--dynamic-list=$(srcdir)/proc-service.list'],[RDYNAMIC="-rdynamic"
                     LDFLAGS="$old_LDFLAGS $RDYNAMIC"
    -                AC_TRY_LINK([], [],
    +                _au_m4_changequote([,])AC_TRY_LINK([], [],
                                 [found="-rdynamic"],
                                 [found="no"
                                  RDYNAMIC=""])])

... which I had to convert manually.

The changes in the generated configure file only contain insignificant
whitespace changes, so that gives confidence that the conversion is
correct.

gdbserver/ChangeLog:

	* configure.ac: Modernize.
	* configure: Re-generate.

Change-Id: Ia769aaec2aafac595504f477da955e91dffa4d8f