configure: handle clang -nopie argument warning

gcc 4.9.2 treats -nopie as an error:

  cc: error: unrecognized command line option ‘-nopie’

clang 3.5.0 treats -nopie as a warning:

  clang: warning: argument unused during compilation: '-nopie'

The causes ./configure to fail with clang:

  ERROR: configure test passed without -Werror but failed with -Werror.

Make the -nopie test use -Werror so that compile_prog works for both gcc
and clang.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-id: 1427324259-1481-2-git-send-email-jsnow@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

configure

@@ -1607,7 +1607,7 @@ EOF
     fi
   fi
 
-  if compile_prog "-fno-pie" "-nopie"; then
+  if compile_prog "-Werror -fno-pie" "-nopie"; then
     CFLAGS_NOPIE="-fno-pie"
     LDFLAGS_NOPIE="-nopie"
   fi