Detecting gated functions w/ AC_CHECK_FUNCS()

[Philip Prindeville]

Hi,

I was wondering how to do discovery of functions like open_memstream() which is 
only exposed by <features.h> when compilation used -D_GNU_SOURCE or 
-D_POSIX_C_SOURCE=200809L.

It might be the case that I compile everything with a certain flag, or I might 
just compile one-off modules such as:

time.o: _ASTCFLAGS+=-D_XOPEN_SOURCE=700

What's best practices for handling situations like this?

How do I bracket a particular AC_CHECK_FUNCS() invocation with defines that 
might be critical?

I tried using:

save_CFLAGS="$CFLAGS"
CFLAGS="CFLAGS -D_POSIX_C_SOURCE=200809L"
AC_CHECK_FUNCS([open_memstream])
CFLAGS="$save_CFLAGS"

But this doesn't seem to make a difference.

Thanks,

-Philip