Squashed commit of the following:

commit db752c0c916f36479b0c5049c671538565294c25
Author: Nikos Armenatzoglou <narmenatzoglou@gmail.com>
Date:   Thu Mar 24 14:01:38 2016 -0700

    Adding check to avoid unit test failures when sizeof(long) = 8

commit 1c39c58279cf73af92811ed1b2e6f2bc22d01414
Author: Shreedhar Hardikar <shardikar@pivotal.io>
Date:   Thu Mar 17 19:42:47 2016 +0000

    --with-codegen-prefix instead of CMAKE_PREFIX_PATH

commit 77978e92444580bc4e84dc3f6b3e366a09e92dba
Author: Shreedhar Hardikar <shardikar@pivotal.io>
Date:   Thu Mar 10 14:57:48 2016 +0000

    Make codegen work with GPDB build system and other minor fixes

      * Update CMakeLists to build SUBSYS.o
      * Update CMakeLists to build release build by default
      * Update CMakeLists to add tests in a loop.
      * Add codegen and cmake specific .gitignore
      * Update license and READMEs
      * Move codegen utils related header files back into src/backend/codegen/include/codegen
Signed-off-by: NNikos Armenatzoglou <nikos.armenatzoglou@gmail.com>

commit 1b61c12f4eb082f49a66c28957c22810e98a5074
Author: Shreedhar Hardikar <shardikar@pivotal.io>
Date:   Wed Mar 16 21:06:28 2016 +0000

    Remove codegen specific gtest source tree and use the one in gpAux.
Signed-off-by: NNikos Armenatzoglou <nikos.armenatzoglou@gmail.com>

commit 13bb2dcf3465be0ef240e316a149b2d8a125a9e9
Author: Shreedhar Hardikar <shardikar@pivotal.io>
Date:   Thu Mar 10 02:51:29 2016 +0000

    Set up GPDB to use codegen utilities

      * Move in codegen utility source and include files to GPDB specific locations.
      * Configure codegen with --enable-codegen and use gpcodegen namespace
Signed-off-by: NNikos Armenatzoglou <nikos.armenatzoglou@gmail.com>

commit 9a4e40f931f71cb7a82e15b51745944116bc10ab
Author: Craig Chasseur <spoonboy42@gmail.com>
Date:   Mon Nov 2 09:10:37 2015 -0800

    Introduce codegen utilties into GPDB.

    Support library for integrating LLVM code-generation and runtime
    compilation with Clang in a larger codebase.
Signed-off-by: NNikos Armenatzoglou <nikos.armenatzoglou@gmail.com>