HADOOP-9350 -add entry to BUILDING.TXT to say 'no java7 on OS/X

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1460634 13f79535-47bb-0310-9956-ffa450edef68

BUILDING.txt

@@ -139,6 +139,15 @@ Create a local staging version of the website (in /tmp/hadoop-site)
 
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+Building on OS/X
+
+----------------------------------------------------------------------------------
+
+Hadoop does not build on OS/X with Java 7.
+see: https://issues.apache.org/jira/browse/HADOOP-9350
+
+----------------------------------------------------------------------------------
+
 Building on Windows
 
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