add var_cond_assign.mk

--- out.make    2015-04-02 16:40:10.734182649 +0900
+++ out.kati    2015-04-02 16:40:10.734182649 +0900
@@ -1,6 +1,6 @@
 === test ===
-echo "FOO BAR"
-FOO BAR
+echo "FOO "
+FOO

 === FILES ===
1 file changed