gdb: modernize configure.ac

gdb: modernize configure.ac

Run autoupdate on configure.ac and adjust the indentation of the result
for better readability.  This removes a bunch of warnings when running
`autoreconf -vf -Wall`.  The changes are:

    * Replace AC_INIT with AC_INIT and no arguments plus
      AC_CONFIG_SRCDIR.
    * Replace AC_ERROR with AC_MSG_ERROR.
    * Replace AC_TRY_LINK with AC_LINK_IFELSE.
    * Replace AC_TRY_COMPILE with AC_COMPILE_IFELSE.
    * Replace AC_HELP_STRING with AS_HELP_STRING.

autoupdate erroneously tries to replace AC_C_LONG_DOUBLE in a comment,
which I reverted manually.

All the changes in the generated configure file are insignificant
whitespaces changes.

gdb/ChangeLog:

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

Change-Id: Ie3a1409c8032a36a6383da964286a46ece9b546e