These are already defined in fcobjsghash.h which is generated from
fcobjshash.gperf. This has been working fine because we were detecting
the len of FC_GPERF_SIZE_T, but it's incorrect in some cases, such as
on Windows, and then we get build errors like:

```
In file included from ../src/fcobjs.c:33:
src/fcobjshash.gperf:28:1: error: conflicting types for 'FcObjectTypeHash'
 "aspect",FC_ASPECT_OBJECT
 ^~~~~~~~~~~~~~~~
../src/fcobjs.c:28:1: note: previous declaration of 'FcObjectTypeHash' was here
 FcObjectTypeHash (register const char *str, register FC_GPERF_SIZE_T len);
 ^~~~~~~~~~~~~~~~
In file included from ../src/fcobjs.c:33:
src/fcobjshash.gperf:192:1: error: conflicting types for 'FcObjectTypeLookup'
../src/fcobjs.c:31:1: note: previous declaration of 'FcObjectTypeLookup' was here
 FcObjectTypeLookup (register const char *str, register FC_GPERF_SIZE_T len);
 ^~~~~~~~~~~~~~~~~~
```

We don't actually need to auto-detect this value in configure if we
let the header define it.