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 <sean@cloudera.com>
Tested-by: Sean Mackrory <sean@cloudera.com>
CDH-18430. Snappy build fails for various reasons on various distros

    Using the build scripts shipped with the official Snappy release, the build
    can occasionally fail due to special characters in the path and race
    conditions, and these failures do not always occur consistently. This
    forces autotools to regenerate the build scripts each time and overwrite
    existing scripts, and works around several problems encountered when
    regenerating on older distros including doc_dir not having a default value
    and initializing libtool a different way.

Change-Id: I326a656d4c3c258e5eb71f69f54eee6b6d434290
(cherry picked from commit acf1fe90264cfda4d72e836e063c32403e35f1e1)
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4066
Reviewed-by: Sean Mackrory <sean@cloudera.com>
Tested-by: Sean Mackrory <sean@cloudera.com>