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.