This avoids propagating -std=c++14 as a compile option
Currently if you use less than CMake 3.8 and you install the project
`-std=c++14` gets added to the INTERFACE_COMPILE_OPTIONS. This forces
users to use C++ 14 or remove the property from the imported target.
The solution is to raise the minimum and use `cxx_std_14`
- Move all install logic inside gsl_install.cmake
- This makes reading the logic easier, and avoids the enable language
issue with `GNUInstallDirs` by having it included after the project
call
- Have all functions inside gsl_functions.cmake
- Use CMake idiom PROJECT_IS_TOP_LEVEL
- Update README.md
jpr42