add compiler flag -Wno-error=incompatible-function-pointer-types

[davidfdriscoll]

On Apple M1, macOS Sonoma 14.3.1, Apple clang version 15.0.0

`CFLAGS='-std=gnu89 -Wno-return-type -Wno-implicit-function-declaration' make LOADLIBES='-ll' failed with the following error message:

genwd.c:49:48: error: incompatible function pointer types passing 'int (gk_word *, gk_word *)' to parameter of type 'int (* _Nonnull)(const void *, const void *)' [-Wincompatible-function-pointer-types]
        qsort(gkforms,formcnt,sizeof * gkforms,CompGkForms);    
                                               ^~~~~~~~~~~

Apparently the warning "incompatible function pointer types" was upgraded to a fatal error in Aug. 2022. This PR adds the compiler flag -Wno-error=incompatible-function-pointer-types that reverts the error to a warning.

Tested locally, project now builds and runs successfully.