configure: Only build the s390-ccw bios if the compiler supports -march=z900

We want to build our s390-ccw bios with -march=z900 so that it also
works with the oldest s390x CPU that we support with TCG. However,
Clang on s390x does not support -march=z900 anymore, so we can not
use this compiler to build the s390-ccw bios. Thus add a proper test
to the configure script to see whether the compiler is usable.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1547470346-18416-1-git-send-email-thuth@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

configure

@@ -5889,8 +5889,12 @@ if test "$cpu" = "ppc64" -a "$targetos" != "Darwin" ; then
   roms="$roms spapr-rtas"
 fi
 
+# Only build s390-ccw bios if we're on s390x and the compiler has -march=z900
 if test "$cpu" = "s390x" ; then
-  roms="$roms s390-ccw"
+  write_c_skeleton
+  if compile_prog "-march=z900" ""; then
+    roms="$roms s390-ccw"
+  fi
 fi
 
 # Probe for the need for relocating the user-only binary.