Choose SINGLE_PRECISION by default in fp_lib.h after manually #erroring when no precision is set. This eliminates cluttering log with lots of errors like "unknown type name 'fp_t'" after the descriptive error message was already generated manually. In this patch, I assume that #error cannot be silenced.
This makes clang-tidy output during differential review of patches more relevant for fp_*_impl.inc while checking macroses even stricter (checking that *exactly* one is defined instead of *at least* one is).
Example output of clang-tidy for fp_add_impl.inc: