Add --enable-cxx14 and --cflags to the configure script.

For now --enable-cxx14 uses -std=c++1y

--cflags should be used when custom configuring ROOT with compiler flags that
affects binary compatibility (like -std=c++1y for example)

This prevents this kind of important customization, historically done
in MyRules.mk, from being 'forgotten' from root-config.

Conflicts:
	config/Makefile.ios
	config/Makefile.ios64
	config/Makefile.iossim
	config/Makefile.linux
	config/Makefile.linuxarm64
	config/Makefile.linuxx8664gcc
	config/Makefile.linuxx8664icc
	config/Makefile.macosx
	config/Makefile.macosx64
	config/root-config.in
	configure
also compile clang/llvm in c++14