COMP: Update BRAINSTools to fix MacOSX and Linux packaging.

This commit fixes a regression originally introduced in r24881 (BUG: Update
BRAINSTools to include fixes for BRAINSFit and BRAINSResample.)

Fixes 0004135

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commit Slicer/BRAINSTools@1a8e84b
Author: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>
Date: Sat Jan 23 05:06:52 2016 -0500

COMP: Force CMAKE_CONFIGURATION_TYPES only when using multi-config generator

This commit fixes a regression first introduced in BRAINSia/BRAINSTools@831eab6 (ENH: Allow
selecting C++11 from CMAKE_CXX_STANDARD) and incorrectly fixed
in BRAINSia/BRAINSTools@8987b5d (BUG: Do not dereference a variable when checking for existance.)

CMAKE_CONFIGURATION_TYPES variable is now forced to a value only
if it has been defined by CMake. This ensures that project (e.g Slicer)
directly adding BRAINSTools source tree can still rely on the
assumption CMAKE_CONFIGURATION_TYPES is set only when multi-configuration
generator are used.

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git-svn-id: http://svn.slicer.org/Slicer4/trunk@24889 3bd1e089-480b-0410-8dfb-8563597acbee