gdb/configure: use AC_MSG_NOTICE not a direct echo call

gdb/configure: use AC_MSG_NOTICE not a direct echo call

After the recent commits, I noticed that GDB's configure script would
still emit two lines even when run in silent mode.  If you touch
gdb/Makefile.in and then run 'make all' in the gdb/ build directory
you'll see this:

    GEN    config.status
  enable_sim = no
  enableval = no

Obviously the 'no' might be 'yes' depending on how you actually
configured GDB.

This is caused by two direct invocations of 'echo' in GDB's
configure.ac script.

In this commit I replace these calls with use of AC_MSG_NOTICE
instead.  Now when configure is run with the --silent command line
option these lines will not be printed.

There should be no changes in the built GDB after this commit.

Approved-By: Simon Marchi <simon.marchi@efficios.com>