solaris: ensure that -D_REENTRANT gets set

Apparently some Solaris' require this for threadsafe
errno.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/configure b/configure
index b6bfe19..ef7be01 100755
--- a/configure
+++ b/configure
@@ -183,6 +183,7 @@
   targetos='OpenBSD'
 elif check_define __sun__ ; then
   targetos='SunOS'
+  CFLAGS="$CFLAGS -D_REENTRANT"
 else
   targetos=`uname -s`
 fi