CMake: unbundle eigen3

Optionally use systems eigen3 library.

T41989 by @hasufell with edits

CMakeLists.txt

@@ -261,6 +261,10 @@ endif()
 # (unix defaults to System OpenJPEG On)
 option(WITH_SYSTEM_OPENJPEG "Use the operating systems OpenJPEG library" OFF)
 
+if(UNIX AND NOT APPLE)
+	option(WITH_SYSTEM_EIGEN3 "Use the systems Eigen3 library" OFF)
+endif()
+
 
 # Modifiers
 option(WITH_MOD_FLUID           "Enable Elbeem Modifier (Fluid Simulation)" ON)
@@ -811,6 +815,14 @@ if(UNIX AND NOT APPLE)
 		endif()
 	endif()
 
+	if(WITH_SYSTEM_EIGEN3)
+		find_package_wrapper(Eigen3)
+		if(NOT EIGEN3_FOUND)
+			message(FATAL_ERROR "Failed finding system Eigen3 version!")
+		endif()
+	endif()
+	# else values are set below for all platforms
+
 	if(WITH_PYTHON)
 		# No way to set py34. remove for now.
 		# find_package(PythonLibs)
@@ -2246,6 +2258,9 @@ if(WITH_IMAGE_REDCODE)
 	set(REDCODE_INC ${REDCODE})
 endif()
 
+if(NOT WITH_SYSTEM_EIGEN3)
+	set(EIGEN3_INCLUDE_DIRS ${CMAKE_SOURCE_DIR}/extern/Eigen3)
+endif()
 
 #-----------------------------------------------------------------------------
 # Configure OpenGL.