Extend the fix from commit 67e2130c96 (Makefiles: Fix
CMAKE_EXPORT_COMPILE_COMMANDS crash with custom compile rule,
2020-11-20, v3.19.1~8^2) to work for the `<FLAGS>` placeholder too.
Also fix the test case to actually enable `EXPORT_COMPILE_COMMANDS`
on the test target.
Fixes: #22261
The new target property `EXPORT_COMPILE_COMMANDS` associated with the
existing global variable can be used to optionally configure targets for
their compile commands to be exported.
Fixes: #19462
