Change to compile better on Microsoft C compiler.

This is a response to some testing problems on the Microsoft C
compiler, see
<http://ppm4.activestate.com/MSWin32-x86/5.14/1400/B/BK/BKB/JSON-Parse-0.21.d/log-20130808T022902.txt>. The
problems seem to stem from using "inline" in the code so I have made
this into a conditionally-defined macro.