Extended Description

A common way to do autoconf-style tests for declarations in headers uses the following approach:

int main(void) { (void)strlcpy; return 0; }

This should fail with unknown identifier errors, but currently passes with a implicit prototype warning for GNUish language modes (i.e. gnu89, gnu99 etc). It fails for "unknown" identifiers like strlcpy2. This is a pretty bad error as it can lead to incorrect autodetection of system headers.