This makes --enable-build-with-cxx be "yes" by default.

One must now configure with --enable-build-with-cxx=no in order to
build with a C compiler.

gdb/ChangeLog:
2016-04-20  Pedro Alves  <palves@redhat.com>

	* build-with-cxx.m4 (GDB_AC_BUILD_WITH_CXX): Default to yes.
	* configure: Renegerate.

gdb/gdbserver/ChangeLog:
2016-04-20  Pedro Alves  <palves@redhat.com>

	* configure: Renegerate.