Disable link-time optimization for debug builds (#1418)

For some reason, on macOS, debug symbols do not seem to be available for
lldb if link-time optimization is enabled. I do not know exactly why
that's the case, but it seems safe and without downsides to just disable
LTO if we're doing a debug build.

Co-authored-by: jcm <butt@butts.com>

nall/GNUmakefile

@@ -155,6 +155,7 @@ endif
 # build optimization levels
 ifeq ($(build),debug)
   symbols = true
+  lto = false
   ifeq ($(cl),true)
     flags += -Od
   else