[C++] Drop -fpermissive hack

Both x86_64 GNU/Linux and x86_64 mingw-w64 build cleanly with
--enable-targets=all.  Let's drop the -fpermissive hack, in order to
let the buildbot catch C++ build regressions for us.

gdb/ChangeLog:
2015-11-19  Pedro Alves  <palves@redhat.com>

	* build-with-cxx.m4 (GDB_AC_BUILD_WITH_CXX): Remove -fpermissive.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2015-11-19  Pedro Alves  <palves@redhat.com>

	* configure: Regenerate.