Add a small bootstrapper to locate build system

Currently this bootstrapper only finds the macros that are located
in the cmake/ tree in this project. Projects that don't carry their
own build system can specify search locations here.

CMakeLists.txt

@@ -17,7 +17,13 @@
 cmake_minimum_required (VERSION 2.8)
 
 # additional search modules
-list (APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/Modules")
+find_path (OPM_MACROS_ROOT cmake/Modules/OpmInit.cmake
+	PATHS ${opm-macros_ROOT}
+	      ${CMAKE_CURRENT_SOURCE_DIR}
+	DOC "Path to build system macros and scripts"
+	NO_DEFAULT_PATH
+	)
+list (APPEND CMAKE_MODULE_PATH "${OPM_MACROS_ROOT}/cmake/Modules")
 
 # not the same location as most of the other projects? this hook overrides
 macro (dir_hook)