More libstdc++-v3 lossage

[Richard Earnshaw]

NetBSD has a minor bug in its header files, which means that if 
_ISOC99_SOURCE is defined stdlib.h is internally inconsistent (I've 
reported this to the NetBSD folks).  Unfortunately src/Makefile.in 
unconditionally defines this on the command line.

However, looking at the ChangeLogs, it appears that the trend now is to 
move defines like this into the os_defines.h header.  So why is this 
define still present in that particular header file?

This isn't a major problem, I can just undef it in the NetBSD version; but 
it does seem a bit inconsistent.

R.