configure: Make epoll_create1 test work around SPARC glibc bug

Work around a SPARC glibc bug which caused the epoll_create1 configure
test to wrongly claim that the function was present. Some versions of
SPARC glibc provided the function in the library but didn't declare
it in the include file; the result is that gcc warns about an implicit
declaration but a link succeeds. So we reference the function as a
value rather than a function call to induce a compile time error
if the declaration was not present.

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

configure

@@ -2225,7 +2225,15 @@ cat > $TMPC << EOF
 
 int main(void)
 {
-    epoll_create1(0);
+    /* Note that we use epoll_create1 as a value, not as
+     * a function being called. This is necessary so that on
+     * old SPARC glibc versions where the function was present in
+     * the library but not declared in the header file we will
+     * fail the configure check. (Otherwise we will get a compiler
+     * warning but not an error, and will proceed to fail the
+     * qemu compile where we compile with -Werror.)
+     */
+    epoll_create1;
     return 0;
 }
 EOF