patch 8.1.0785: depending on the configuration some functions are unused

Problem:    Depending on the configuration some functions are unused.
Solution:   Add more #ifdefs, remove unused functions. (Dominique Pelle,
            closes #3822)

src/proto/regexp.pro

@@ -1,6 +1,5 @@
 /* regexp.c */
 int re_multiline(regprog_T *prog);
-int re_lookbehind(regprog_T *prog);
 char_u *skip_regexp(char_u *startp, int dirc, int magic, char_u **newp);
 int vim_regcomp_had_eol(void);
 void free_regexp_stuff(void);