That's not a function pointer, it's a function declaration. The A_ is a macro defined somewhere in the source. It looks to be used to create code that builds with both K&R compilers and modern compilers.
Prototypes are a somewhat new feature in C, and so really old compilers don't support them. However, it'd be stupid to not use prototypes in new code because they're really useful. Thus people who, for some reason, care about older (roughly pre-1990) compilers, use a trick like the following to do this:
The second version is clearly superior, but if your compiler doesn't support it, you've got to use the inferior version.
#if __STDC__ == 1
#define PROTO(p) p
#define PROTO(p) ()
int f PROTO((const char *));
/* Under older compilers (__STDC__ is not 1), you get: */
int f ();
/* Under modern compilers (__STDC__ is 1), you get: */
int f (const char *);
To see exactly what the macro in your sources looks like, grep for A_ (it's probably in a header somewhere).