Use `strcmp` in instead of `gvrender_comparestr`.

I saw John working on this and had a look at it as well. bsearch is used in this function to find a string in a string array. It's not needed to pass the address of tok to the function, because tok is already a pointer. gvrender_comparestr should then cast the values to (const char) instead of *(char**), because s1 and s2 are strings, not pointers to a string. With this change the function only calls strcmp, so passing strcmp to bsearch has the same effect and gvrender_comparestr can be removed. This gets rid of all the -Wcast-qual warnings.