Install directories for same headers should not be specified multiple times

[barracuda156]

Consider a case when a user wants to install headers into ${prefix}/include/GFTL (as opposed to dumping them into a common dir and promoting a chaos there). And then, inside GFTL there are V1 and V2 (without an unnecessary another include in between). There seems to be no trivial way to do that, and even patching CMake files requires multiple iterations, because settings in one CMakeLists file are ignored due to the same being redefined elsewhere.
If one does not notice what the build system does, everything gets coerced into ${prefix}/GFTL-${version}.

[tclune]

I am a bit confused by this request. AFAIK, gFTL is installing in a canonical fashion. I am not strongly opposed to further generalization, but we have found that such changes often unintentionally break other build choices, and thus need to be thoroughly vetted.

@mathomp4 - FYI.