Feature Conditional for M_PI

Hi

I was porting some code from Linux to Cygwin and came across this. M_PI in
math.h is defined by POSIX as part of XSI. It does not appear to be part of
C99 or C++03.  I have this cut down to show the problem:

==========================
#include <math.h>

double pi = M_PI;
==========================

And this script to try various feature defines and compilers:

===========================
GCC=${GCC:-g++}

${GCC} -c m.c
${GCC} -D_XOPEN_SOURCE=700 -c m.c
${GCC} -D_POSIX_C_SOURCE=200809L -c m.c
${GCC} -D_XOPEN_SOURCE=700 -c -D_POSIX_C_SOURCE=200809L -c m.c
===========================

All of those compiler invocations work on Linux but the third one does not
work on Cygwin or RTEMS which use newlib.

Is the proper thing to do to add  -D_XOPEN_SOURCE=700 when compiling this
program?

Just curious if Linux is defining _XOPEN_SOURCE by default and newlib
doesn't. Not sure what's the right answer. So asking.

Thanks

--joel