Build: Enable -Wconditionally-supported on GCC

We want to have warnings if we use some implementation-specific features of GCC
to be able to keep compatibility with clang.

cmake/compiler/gcc/settings.cmake

@@ -1,6 +1,7 @@
 target_compile_options(zm-warning-interface
   INTERFACE
     -Wall
+    -Wconditionally-supported
     -Wextra
     -Wformat-security
     -Wno-cast-function-type