Modernize configure.ac's

Using AC_OUTPUT with arguments has been deprecated for some time in
autoconf, even in version 2.64, which we are using.  This change should
not affect functionality.

I also removed the "exit 0"'s, they shouldn't be necessary.

gdb/ChangeLog:

	* configure.ac: Use AC_CONFIG_FILES instead of passing arguments
	to AC_OUTPUT.  Remove "exit 0" at the end.
	* configure: Regenerate.

gdb/testsuite/ChangeLog:

	* configure.ac: Use AC_CONFIG_FILES instead of passing arguments
	to AC_OUTPUT.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:

	* configure.ac: Use AC_CONFIG_FILES instead of passing arguments
	to AC_OUTPUT.
	* configure: Regenerate.