configure: avoid compiler warning in pipe2 detection

When building qemu-kvm for openSUSE:Factory, I am getting a
warning in the pipe2 detection performed by configure, which
prevents using --enable-werror.

Change detection code to use return value of pipe2.

Signed-off-by: Bruce Rogers <brogers@suse.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

configure

@@ -2399,8 +2399,7 @@ cat > $TMPC << EOF
 int main(void)
 {
     int pipefd[2];
-    pipe2(pipefd, O_CLOEXEC);
-    return 0;
+    return pipe2(pipefd, O_CLOEXEC);
 }
 EOF
 if compile_prog "" "" ; then