Yes, these too many path come from official ebuild in gentoo (outdated). As you said I thing it has to be "simplified" at the path level. But this is another story..... It's gentoo dev/user problem.
Here I think there is a problem with generated install script from subviews branch.
In what follow there's no ebuild nor distribution specific. I directly get source, compile and install manually (git, cmake, make)
here are the cmake arguments used : cmake .. -DSDL_BACKEND=SDL2 -DCMAKE_BUILD_TYPE=Debug -DOPENGL_BACKEND=OpenGL
at 'make install' (system wide, so /usr/local/), '/usr/local/share/gemrb/Shaders' is not here with subviews branch
If I do the same (same cmake args) with master branch, the directory is here.
Theses are the lines in master (/build/gemrb/cmake_install.cmake) after build which are not in with subviews branch :
If I manually add them before 'make install' in '/build/gemrb/cmake_install.cmake' (subviews branch), 'Shaders' directory get installed.
I hope it's more clear ?