Fix GH-16199: GREP_HEADER() is broken

This also fixes the libxml version check when the libxml/xmlversion.h
is located elsewhere than libxml2 include directory.

Closes GH-15619.

Author: petk

NEWS

@@ -71,6 +71,9 @@ PHP                                                                        NEWS
   . Fixed bug GH-15169 (stack overflow when var serialization in
     ext/standard/var). (nielsdos)
 
+- Windows:
+  . Fixed bug GH-16199 (GREP_HEADER() is broken). (Peter Kokot)
+
 26 Sep 2024, PHP 8.4.0RC1
 
 - BcMath:

win32/build/confutils.js

@@ -965,14 +965,19 @@ function GREP_HEADER(header_name, regex, path_to_check)
 
 	if (!c) {
 		/* look in the include path */
+		if (path_to_check == null) {
+			path_to_check = php_usual_include_suspects;
+		} else {
+			path_to_check += ";" + php_usual_include_suspects;
+		}
 
 		var p = search_paths(header_name, path_to_check, "INCLUDE");
 		if (typeof(p) == "string") {
-			c = file_get_contents(p);
+			c = file_get_contents(p + "\\" + header_name);
 		} else if (p == false) {
 			p = search_paths(header_name, PHP_EXTRA_INCLUDES, null);
 			if (typeof(p) == "string") {
-				c = file_get_contents(p);
+				c = file_get_contents(p + "\\" + header_name);
 			}
 		}
 		if (!c) {