Consider drive letters when checking for absolute paths on Windows.

This still requires you to specify paths with forward slashes instead of
backslashes on Windows, due to many hardcoded checks for '/'.
Fortunately, the Windows user APIs all support forward slashes too.

Signed-off-by: Colin Finck <mail@colinfinck.de>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

srcpos.c

@@ -88,6 +88,26 @@ static char *shorten_to_initial_path(char *fname)
 	return NULL;
 }
 
+/**
+ * Returns true if the given path is an absolute one.
+ *
+ * On Windows, it either needs to begin with a forward slash or with a drive
+ * letter (e.g. "C:").
+ * On all other operating systems, it must begin with a forward slash to be
+ * considered an absolute path.
+ */
+static bool is_absolute_path(const char *path)
+{
+#ifdef WIN32
+	return (
+		path[0] == '/' ||
+		(((path[0] >= 'A' && path[0] <= 'Z') || (path[0] >= 'a' && path[0] <= 'z')) && path[1] == ':')
+	);
+#else
+	return (path[0] == '/');
+#endif
+}
+
 /**
  * Try to open a file in a given directory.
  *
@@ -103,7 +123,7 @@ static char *try_open(const char *dirname, const char *fname, FILE **fp)
 {
 	char *fullname;
 
-	if (!dirname || fname[0] == '/')
+	if (!dirname || is_absolute_path(fname))
 		fullname = xstrdup(fname);
 	else
 		fullname = join_path(dirname, fname);