auto merge of #9385 : alexcrichton/rust/actually-ndebug, r=pnkfelix

Turns out that even if the default is "enabled", that doesn't mean that the
CFG_ENABLE_DEBUG variable will be defined. Instead, test whether
CFG_DISABLE_DEBUG is defined and disable debug things if that's the case.

Makefile.in

@@ -100,12 +100,13 @@ else
   CFG_RUSTC_FLAGS += -O --cfg rtopt
 endif
 
-ifdef CFG_ENABLE_DEBUG
-  $(info cfg: enabling more debugging (CFG_ENABLE_DEBUG))
-  CFG_GCCISH_CFLAGS += -DRUST_DEBUG
-else
+ifdef CFG_DISABLE_DEBUG
   CFG_RUSTC_FLAGS += --cfg ndebug
   CFG_GCCISH_CFLAGS += -DRUST_NDEBUG
+else
+  $(info cfg: enabling more debugging (CFG_ENABLE_DEBUG))
+  CFG_RUSTC_FLAGS += --cfg debug
+  CFG_GCCISH_CFLAGS += -DRUST_DEBUG
 endif
 
 ifdef SAVE_TEMPS