Getting nmsg to build on mac

I put in some workaround code for clock_gettime since mac doesn't have librt (http://stackoverflow.com/questions/5167269/clock-gettime-alternative-in-mac-os-x).

I expanded the LOC_SNPRINTF and the SNP_ARG macros because the preprocessor on mac was confused by the number of supplied arguments.

Also, I'm not exactly certain what the exact consequences are of removing the unknown AM_LDFLAGS for ld on mac.