CMake: unbundle eigen3

Optionally use systems eigen3 library.

T41989 by @hasufell with edits

build_files/cmake/Modules/FindEigen3.cmake

@@ -0,0 +1,56 @@
+# - Find Eigen3 library
+# Find the native Eigen3 includes and library
+# This module defines
+#  EIGEN3_INCLUDE_DIRS, where to find spnav.h, Set when
+#                        EIGEN3_INCLUDE_DIR is found.
+#  EIGEN3_ROOT_DIR, The base directory to search for Eigen3.
+#                    This can also be an environment variable.
+#  EIGEN3_FOUND, If false, do not try to use Eigen3.
+#
+#=============================================================================
+# Copyright 2015 Blender Foundation.
+#
+# Distributed under the OSI-approved BSD License (the "License");
+# see accompanying file Copyright.txt for details.
+#
+# This software is distributed WITHOUT ANY WARRANTY; without even the
+# implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+# See the License for more information.
+#=============================================================================
+
+# If EIGEN3_ROOT_DIR was defined in the environment, use it.
+IF(NOT EIGEN3_ROOT_DIR AND NOT $ENV{EIGEN3_ROOT_DIR} STREQUAL "")
+  SET(EIGEN3_ROOT_DIR $ENV{EIGEN3_ROOT_DIR})
+ENDIF()
+
+SET(_eigen3_SEARCH_DIRS
+  ${EIGEN3_ROOT_DIR}
+  /usr/local
+  /sw # Fink
+  /opt/local # DarwinPorts
+  /opt/csw # Blastwave
+)
+
+FIND_PATH(EIGEN3_INCLUDE_DIR
+  NAMES
+    # header has no '.h' suffix
+    Eigen/Eigen
+  HINTS
+    ${_eigen3_SEARCH_DIRS}
+  PATH_SUFFIXES
+    include/eigen3
+)
+
+# handle the QUIETLY and REQUIRED arguments and set EIGEN3_FOUND to TRUE if 
+# all listed variables are TRUE
+INCLUDE(FindPackageHandleStandardArgs)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(Eigen3 DEFAULT_MSG
+    EIGEN3_INCLUDE_DIR)
+
+IF(EIGEN3_FOUND)
+  SET(EIGEN3_INCLUDE_DIRS ${EIGEN3_INCLUDE_DIR})
+ENDIF(EIGEN3_FOUND)
+
+MARK_AS_ADVANCED(
+  EIGEN3_INCLUDE_DIR
+)