Fix: [Cygwin] Fix missing declaration of strdup()

src/depend/depend.cpp: In constructor 'File::File(const char*)':
src/depend/depend.cpp:170:19: error: 'strdup' was not declared in this scope
   this->dirname = strdup(filename);
                   ^~~~~~

Signed-off-by: Joe Stringer <joe@wand.net.nz>

src/depend/depend.cpp

@@ -107,6 +107,23 @@ static char *strecat(char *dst, const char *src, const char *last)
 	return strecpy(dst, src, last);
 }
 
+#if defined(__CYGWIN__)
+/**
+ * Version of strdup copied from glibc.
+ * Duplicate S, returning an identical malloc'd string.
+ * @param s The string to duplicate.
+ */
+char *
+strdup (const char *s)
+{
+	size_t len = strlen(s) + 1;
+	void *n = malloc(len);
+
+	if (n == NULL) return NULL;
+	return (char *) memcpy(n, s, len);
+}
+#endif
+
 /**
  * Version of the standard free that accepts const pointers.
  * @param ptr The data to free.