Add C89 support

Allow compilation from C89 compilers.

src/memory.c

@@ -0,0 +1,23 @@
+/*
+ * Memory allocator for TinyGL
+ */
+#include "zgl.h"
+
+/* modify these functions so that they suit your needs */
+
+void gl_free(void *p)
+{
+    free(p);
+}
+
+void *gl_malloc(int size)
+{
+    return malloc(size);
+}
+
+void *gl_zalloc(int size)
+{
+    return calloc(1, size);
+}
+
+/* --- end of MEMORY_C --- */