CDH-20407. Make it possible for Impala build to point at arbitrary locations for Boost libraries.
CDH-20407. Make it possible for Impala build to point at arbitrary locations for Boost libraries.

    Before, System directories were being included with higher precedence than other
    directories returned by find_* modules. Such modules should be checking in System
    locations anyway and the precedence should be honored (specifically with Boost, as many
    distros do not ship an appropriate version of Boost libraries). This also works around
    a bug in some CMake versions where System locations always take precedence over
    BOOST_ROOT, as documented here:

        http://www.cmake.org/pipermail/cmake/2011-November/047533.html

Change-Id: I5f658ce1121f99f4cd5f6e51944e2951c6cac544
(cherry picked from commit 0012cce030da8db80ea9208d1683640e0d9d7e83)
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4065
Reviewed-by: Sean Mackrory <d7e19930cc1f42c2d0781f4d9e6f1fe5891bf9cf@cloudera.com>
Tested-by: Sean Mackrory <d7e19930cc1f42c2d0781f4d9e6f1fe5891bf9cf@cloudera.com>

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