cmake: updated to 3.15.2

CMake 3.15.2
* Swift: Restore support for enabling with INTERFACE libraries
* VS: Fix mapping of `-Qspectre-` flag
* source_group: Fix regression in relative FILES
* clang: Restore support for clang-cl on non-Windows hosts
* fileapi: Fix codemodel target install destination for cross-dir rules
* clang: Work around toolchain file use of internal CMake variables
* Help: Add 3.15.2 release notes
* FindGLEW: Fix macOS library suffix selection
* FindGLEW: Add required OpenGL dependency in macOS
* FindGLEW: Fix typo in verbose log message
* find_package: Fix prefer-config mode to not fail on missing optional package

(adam)