Add -DNDEBUG to libuv build

[nealrichardson]

I can confirm this is working because I now see in the libuv build:

src/unix/darwin.c:214:17: warning: variable 'kr' set but not used [-Wunused-but-set-variable]
  kern_return_t kr;
                ^

AFAICT kr is only used in assert() statements that NDEBUG removes. This appears not to have an affect on the R CMD check results.

[CLAassistant]

All committers have signed the CLA.

[jcheng5]

LGTM but I'm going to hold off on merging until we know whether we want to roll back the 1.6.10 changes for the immediate release, or roll forward with these fixes.