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.