Refining compiler warnings to detect race conditions

• adding -Werror=unused-result to ensure that, e.g., lock_guards without variable names are not being optimized out.

This is enabled by default on GCC 14 (Ubuntu 24) and revealed race conditions such as in robot-api@2772f062

/cc @dreher

assigned to @reister

see also: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html