configure: Check whether makecontext() is a glibc stub function

On some systems (notably ARM Linux) glibc provides implementations
of makecontext(), getcontext() and friends which are stubs which
always return failure. Make the configure test for makecontext()
also check for the presence of the __stub_makecontext macro which
indicates the presence of these stubs, so we can avoid trying to use
them and fall back to a different coroutine implementation instead.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

configure

@@ -2723,11 +2723,15 @@ fi
 
 ##########################################
 # check if we have makecontext
+# (and that it's not a glibc stub which always returns -1)
 
 ucontext_coroutine=no
 if test "$darwin" != "yes"; then
   cat > $TMPC << EOF
 #include <ucontext.h>
+#ifdef __stub_makecontext
+#error Ignoring glibc stub makecontext which will always fail
+#endif
 int main(void) { makecontext(0, 0, 0); return 0; }
 EOF
   if compile_prog "" "" ; then