gdb/configure --help: suggest --disable-build-with-cxx instead of --enable...

gdb/configure --help: suggest --disable-build-with-cxx instead of --enable...

We build by default with a C++ compiler, but "configure --help" still
says "--enable-build-with-cxx", which hints that it is by default
disabled.  Update the --help text.

gdb/ChangeLog:
2016-08-05  Pedro Alves  <palves@redhat.com>

	* build-with-cxx.m4: Change help string to be in terms of
	--disable-build-with-cxx.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2016-08-05  Pedro Alves  <palves@redhat.com>

	* configure: Regenerate.