make sure the c++11/y flag is only appended once
Add --cxxflags.  Put user flags last so they can win.

--cflags is passed to the c compiler. --cxxflags to the c++ compiler.  So that the user has a chance to override
a default flag we must be it at the end.

Conflicts:
	config/Makefile.aix5
	config/Makefile.aixgcc
	config/Makefile.alphacxx6
	config/Makefile.alphagcc
	config/Makefile.freebsd4
	config/Makefile.freebsd5
	config/Makefile.freebsd7
	config/Makefile.hpuxacc
	config/Makefile.hpuxgcc
	config/Makefile.hpuxia64acc
	config/Makefile.hurddeb
	config/Makefile.ios
	config/Makefile.ios64
	config/Makefile.iossim
	config/Makefile.linux
	config/Makefile.linuxalphagcc
	config/Makefile.linuxarm
	config/Makefile.linuxarm64
	config/Makefile.linuxhppa
	config/Makefile.linuxia64ecc
	config/Makefile.linuxia64gcc
	config/Makefile.linuxicc
	config/Makefile.linuxmips
	config/Makefile.linuxmips64
	config/Makefile.linuxppc64gcc
	config/Makefile.linuxppcgcc
	config/Makefile.linuxx8664gcc
	config/Makefile.linuxx8664icc
	config/Makefile.linuxx8664k1omicc
	config/Makefile.lynxos
	config/Makefile.macosx
	config/Makefile.macosx64
	config/Makefile.macosxicc
	config/Makefile.openbsd
	config/Makefile.sgicc
	config/Makefile.sgicc64
	config/Makefile.sgin32gcc
	config/Makefile.solaris
	config/Makefile.solaris64CC5
	config/Makefile.solarisCC5
	config/Makefile.win32
	config/Makefile.win32gcc
	config/Makefile.win64gcc
	configure
finish adding userconf_cxxflags
enable C++14 on more platforms