build: fix unsetting of variables in `add_compile_flags` macro

`add_compile_flags` macro relied on undefined behaviour: instead of
unsetting the `_lang` and `_flag` variables, it tried to unset their
values — fix this.

NO_CHANGELOG=build
NO_DOC=build
NO_TEST=build

cmake/utils.cmake

@@ -2,8 +2,8 @@ macro(add_compile_flags langs)
     foreach(_lang ${langs})
         string (REPLACE ";" " " _flags "${ARGN}")
         set ("CMAKE_${_lang}_FLAGS" "${CMAKE_${_lang}_FLAGS} ${_flags}")
-        unset (${_lang})
-        unset (${_flags})
+        unset (_lang)
+        unset (_flags)
     endforeach()
 endmacro(add_compile_flags)