Proper pattern rules (a'la GNU make)

[xparq]

• [x] First, just add inf. rules with paths (and introduce the approximate syntax, with the usual % as a placeholder, for (always) the target's name). -> #1
• [ ] Next, get the GNU syntax (target%pattern : src%pattern) accepted & recognized as inf. rule
• [ ] And then make it actually work, too...

[xparq]

It would be very likely useful to generalize this part out of the static modify_words in input.c (maybe straight to utils.c?):

if (find_pref) {
    // get length of find prefix, e.g: src/
    find_pref_len = strlen(find_pref);
    // get length of find suffix, e.g: .c
    find_suff_len = lenf - find_pref_len - 1;
}

s = copy = xstrdup(val);
while ((word = gettok(&s)) != NULL) {
    newword = NULL;
    if (IF_FEATURE_MAKE_POSIX_202X(find_pref != NULL ||)
            lenf != 0 || lenr != 0) {
        size_t lenw = strlen(word);
#if ENABLE_FEATURE_MAKE_POSIX_202X
        // This code implements pattern macro expansions:
        //    https://austingroupbugs.net/view.php?id=519
        //
        // find: <prefix>%<suffix>
        // example: src/%.c
        //
        // For a pattern of the form:
        //    $(string1:[op]%[os]=[np][%][ns])
        // lenf is the length of [op]%[os].  So lenf >= 1.
        if (find_pref != NULL && lenw + 1 >= lenf) {
            // If prefix and suffix of word match find_pref and
            // find_suff, then do substitution.
            if (strncmp(word, find_pref, find_pref_len) == 0 &&
                    strcmp(word + lenw - find_suff_len, find_suff) == 0) {
                // replace: <prefix>[%<suffix>]
                // example: build/%.o or build/all.o (notice no %)
                // If repl_suff is NULL, replace whole word with repl_pref.
                if (!repl_suff) {
                    word = newword = xstrdup(repl_pref);
                } else {
                    word[lenw - find_suff_len] = '\0';
                    word = newword = xconcat3(repl_pref,
                                word + find_pref_len, repl_suff);
                }
            }
        } else
#endif
        if (lenw >= lenf && strcmp(word + lenw - lenf, find_suff) == 0) {
            word[lenw - lenf] = '\0';
            word = newword = xconcat3(word, repl_suff, "");
        }
    }