Hector_slam not find eigen

eigen library location, often found cmake can not find, modify cmake, the link below

https://github.com/tu-darmstadt-ros-pkg/hector_slam/commit/d9e44fb85087cdf40d4fe702082701b2882eefea

			find_package(Eigen3)
			if(NOT EIGEN3_FOUND)
			# Fallback to cmake_modules
			find_package(cmake_modules REQUIRED)
			find_package(Eigen REQUIRED)
			set(EIGEN3_INCLUDE_DIRS ${EIGEN_INCLUDE_DIRS})
			set(EIGEN3_LIBRARIES ${EIGEN_LIBRARIES}) # Not strictly necessary as Eigen is head only
			# Possibly map additional variables to the EIGEN3_ prefix.
			else()
			set(EIGEN3_INCLUDE_DIRS ${EIGEN3_INCLUDE_DIR})
			endif()