Raise CMake min to 3.14 (#1083)

- Explicitly test package manager support with CMake 3.14 and Ninja
- Remove gsl_install.cmake
- Simplify installation code

cmake/gsl_install.cmake

@@ -1,26 +0,0 @@
-include(GNUInstallDirs)
-include(CMakePackageConfigHelpers)
-
-# Install header files
-install(DIRECTORY "${PROJECT_SOURCE_DIR}/include/gsl" DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
-
-set(export_name "Microsoft.GSLConfig")
-set(namespace "Microsoft.GSL::")
-set(cmake_files_install_dir ${CMAKE_INSTALL_DATADIR}/cmake/Microsoft.GSL)
-
-# Add find_package() support
-target_include_directories(GSL INTERFACE $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
-install(TARGETS GSL EXPORT ${export_name})
-install(EXPORT ${export_name} NAMESPACE ${namespace} DESTINATION ${cmake_files_install_dir})
-export(TARGETS GSL NAMESPACE ${namespace} FILE ${export_name}.cmake)
-
-set(gls_config_version "${CMAKE_CURRENT_BINARY_DIR}/Microsoft.GSLConfigVersion.cmake")
-
-# Add find_package() versioning support
-if(${CMAKE_VERSION} VERSION_LESS "3.14.0")
-    write_basic_package_version_file(${gls_config_version} COMPATIBILITY SameMajorVersion)
-else()
-    write_basic_package_version_file(${gls_config_version} COMPATIBILITY SameMajorVersion ARCH_INDEPENDENT)
-endif()
-
-install(FILES ${gls_config_version} DESTINATION ${cmake_files_install_dir})