Fix build of swrast only without libdrm

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Daniel Martin <consume.noise@gmail.com>

configure.ac

@@ -1095,7 +1095,7 @@ if test "x$enable_dri" = xyes; then
     fi
 
     # if we are building any dri driver other than swrast or using the dri state tracker ...
-    if test -n "$DRI_DIRS" -a x"$DRI_DIRS" != xswrast || test "x$enable_dri" = xyes; then
+    if test -n "$DRI_DIRS" -a x"$DRI_DIRS" != xswrast -a "x$enable_dri" = xyes; then
         # ... libdrm is required
         if test "x$have_libdrm" != xyes; then
             AC_MSG_ERROR([DRI drivers requires libdrm >= $LIBDRM_REQUIRED])