Use alloca() for dynamic storage in the draft makefile.

This function is available on any modern linux and msvc. So while
not strictly part of C89, it's a much better default than the
non-thread-safe global pseudo stack.

Makefile.draft

@@ -7,7 +7,7 @@
 # VAR_ARRAYS: Use C99 variable-length arrays for stack allocation
 # USE_ALLOCA: Use alloca() for stack allocation
 # If none is defined, then the fallback is a non-threadsafe global array
-#CFLAGS += -DUSE_ALLOCA
+CFLAGS += -DUSE_ALLOCA
 #CFLAGS += -DVAR_ARRAYS
 
 # These options affect performance