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Hello John & Steven,
Thanks for ur help.
I was able to resolve the issue.
As per the link
http://gcc.gnu.org/onlinedocs/cpp/Search-Path.html
"All the directories named by -I are searched, in
left-to-right order, before the default directories.
The only exception is when dir is already searched by
default. In this case, the option is ignored and the
search order for system directories remains
unchanged."
Since we have "boost" directory at default
path:/usr/include/boost, it used to look
/usr/include/boost before
-I