Proper pattern rules (a'la GNU make)

[xparq]

• First, just add inf. rules with paths (and introduce the approximate syntax, with the usual % as a placeholder, for (always) the target's name). -> Path-aware inference rules #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, "");
		}
	}