Configure: the --build= option.

If set, its value is output in "nginx -v" and in the error log.
diff --git a/auto/configure b/auto/configure
index d7d8189..e8929b8 100755
--- a/auto/configure
+++ b/auto/configure
@@ -108,4 +108,8 @@
 have=NGX_USER value="\"$NGX_USER\"" . auto/define
 have=NGX_GROUP value="\"$NGX_GROUP\"" . auto/define
 
+if [ ".$NGX_BUILD" != "." ]; then
+    have=NGX_BUILD value="\"$NGX_BUILD\"" . auto/define
+fi
+
 . auto/summary