CMake cleanup

[jpr42]

• 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]

I tested that find_package still works as expected by using the tests directory which invokes find_package(Microsoft.GSL REQUIRED) if it consumed as a top level project.

• This makes reading the logic easier, and avoids the enable language issue with GNUInstallDirs by having it included after the project call

By issue I just mean how the logic was a bit awkward. Since the guidelineSupportLibrary.cmake relied on GNUInstallDirs but couldn't directly include itself being included before a language is enabled.

include(gsl_functions)

project(GSL
    VERSION 4.0.0
    LANGUAGES CXX // Language is enabled here <- Now you can use GNUInstallDirs
)

Now GNUInstallDirs is included by gsl_install.cmake which handles all the installation.

[dmitrykobets-msft]

Awesome, thanks a lot @jpr42 for this cleanup.