Install config mode CMake module at installation

find_package only searches for FindXxx-modules in the path specified by
CMAKE_MODULE_PATH; other paths are supposed to contain config-mode
modules (much like pkgconfig).

Generate the config-mode module in the installation directory using the
same template as is used for the one in the build directory, only
selectively swapping some directories in the relevant variables.

cmake/Modules/OpmProject.cmake

@@ -0,0 +1,42 @@
+# - Helper routines for opm-core like projects
+
+# installation of CMake modules to help user programs locate the library
+function (opm_cmake_config name)
+  #  replace the build directory with the target directory in the
+  # variables that contains build paths
+  string (REPLACE
+	"${PROJECT_SOURCE_DIR}"
+	"${CMAKE_INSTALL_PREFIX}/include"
+	${name}_INCLUDE_DIRS
+	"${${name}_INCLUDE_DIRS}"
+	)
+  string (REPLACE
+	"${PROJECT_BINARY_DIR}/lib"
+	"${CMAKE_INSTALL_PREFIX}/lib"
+	${name}_LIBRARY
+	"${${name}_LIBRARY}"
+	)
+  string (REPLACE
+	"${PROJECT_BINARY_DIR}/lib"
+	"${CMAKE_INSTALL_PREFIX}/lib"
+	CMAKE_LIBRARY_OUTPUT_DIRECTORY
+	"${CMAKE_LIBRARY_OUTPUT_DIRECTORY}"
+	)
+  # create a config mode file which targets the install directory instead
+  # of the build directory (using the same input template)
+  configure_file (
+	${PROJECT_SOURCE_DIR}/${name}-config.cmake.in
+	${PROJECT_BINARY_DIR}/${name}-install.cmake
+	@ONLY
+	)
+  configure_vars (
+	FILE CMAKE "${PROJECT_BINARY_DIR}/${name}-install.cmake"
+	APPEND "${${name}_CONFIG_VARS}"
+	)
+  # this file gets copied to the final installation directory
+  install (
+	FILES ${PROJECT_BINARY_DIR}/${name}-install.cmake
+	DESTINATION share/cmake/${name}
+	RENAME ${name}-config.cmake
+	)
+endfunction (opm_cmake_config name)