Avoid if() quoted auto-dereference

When testing CMAKE_<LANG>_COMPILER_ID values, do not explicitly
dereference or quote the variable. We want if() to auto-dereference the
variable and not its value. Also replace MATCHES with STREQUAL where
equivalent.
Merge topic 'cmake-cmp0054-warnings' into next

7ea1da1a Avoid if() quoted auto-dereference