It looks like to be strictly conforming, you can't just use a series of flags because neither C 89 or 99 guarantee that sig_atomic_t is read/write round-trip safe in signal handlers. In other words, if you read, are pre-empted by another signal, and then write, you may clobber what the other signal handler did to the variable. So you need atomics, which are C11....
What I would suggest instead at least for an initial approach is:
1. A struct of volatile bools statically stored
These would be implemented as sig_atomic_t which is defined in C89 but has no atomic operators other than writing the full value.
2. macros for accessing/setting/clearing flags
3. Consistent use of these macros throughout the codebase.
To make your solution work it looks like we'd need C11 atomics which would be nice and maybe at some point we decide to allow newer feature, or we could wrap this itself in checks for C11 features and provide atomic flags in the future. It seems that the above solution would strictly comply to C89 and pose no concurrency issues.