configure: Fix build with capabilities

Since commit 417c9d72d48275d19c60861896efd4962d21aca2 all configure tests
normally run with -Werror. Some of these tests now fail because they
raised a compiler warning.

This patch fixes support for capabilities.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

configure

diff --git a/configure b/configure
index c65b5f6..309aeac 100755
--- a/configure
+++ b/configure
@@ -2084,7 +2084,7 @@
   cat > $TMPC <<EOF
 #include <stdio.h>
 #include <sys/capability.h>
-int main(void) { cap_t caps; caps = cap_init(); }
+int main(void) { cap_t caps; caps = cap_init(); return caps != NULL; }
 EOF
   if compile_prog "" "-lcap" ; then
     cap=yes