Try singular name if plural not defined

In CMake "canon" (i.e. tutorial on Kitware's website) it is stated that
the _DIR suffix should be used for the directory of the library alone,
whereas _DIRS are entire list of paths necessary to compile (i.e.
dependencies are included).

Eigen3 only uses the singular form (since it doesn't have any
dependencies); it should really be copied to the plural form, but it may
be advantageous to have the find-module included unchanged, so we put in
an extra effort to find it here.

cmake/Modules/OpmFind.cmake

@@ -158,6 +158,10 @@ macro (find_and_append_package_to prefix name)
 		elseif (DEFINED ${NAME}_INCLUDE_PATH)
 		  list (APPEND ${prefix}_INCLUDE_DIRS ${${NAME}_INCLUDE_PATH})
 		endif (DEFINED ${name}_INCLUDE_PATH)
+		# some packages define only _DIR and not _DIRS (Hi, Eigen3!)
+		if (NOT DEFINED ${name}_INCLUDE_DIRS)
+		  list (APPEND ${prefix}_INCLUDE_DIRS ${${prefix}_INCLUDE_DIR})
+		endif (NOT DEFINED ${name}_INCLUDE_DIRS)
 	  endif ("${var}" STREQUAL "INCLUDE_DIRS")
 	  # cleanup lists
 	  if ("${var}" STREQUAL "LIBRARIES")