A useful enhancement could be the ability to install multiple versions of the standard library into the same location. While there is nothing in the makefiles that would prevent such an installation, a problem arises with the config.h file in the include sub directory of the install directory. Only one config.h file can reside in this location at a time, and different versions (such as those generated when characterizations are run with different compilers) may differ.
One problem that needs to be considered in implementing (or deciding not to implement) this request is the situation where a config.h file hasn't been generated for a particular os/compiler/buildtype/buildtag.
Another problem to consider is when you have the same os/buildtype/buildtag, but different compilers, and one or both compilers support features that the other doesn't.