Write version file so user can specify lower bound

With the version file present, a user program can depend on a certain
version of the library.

CMakeLists.txt

@@ -138,6 +138,12 @@ configure_vars (
 	FILE CMAKE "${PROJECT_BINARY_DIR}/opm-core-config.cmake"
 	APPEND "${opm-core_CONFIG_VARS}"
 	)
+include (WriteBasicConfigVersionFile)
+write_basic_config_version_file (
+	"${PROJECT_BINARY_DIR}/opm-core-config-version.cmake"
+	VERSION ${opm-core_VERSION}
+	COMPATIBILITY SameMajorVersion
+	)
 
 ### installation ###
 string (LENGTH ${PROJECT_SOURCE_DIR} _prefix_length)

cmake/Modules/OpmProject.cmake

@@ -39,4 +39,9 @@ function (opm_cmake_config name)
 	DESTINATION share/cmake/${name}
 	RENAME ${name}-config.cmake
 	)
+  # assume that there exists a version file already
+  install (
+	FILES ${PROJECT_BINARY_DIR}/${name}-config-version.cmake
+	DESTINATION share/cmake/${name}
+	)
 endfunction (opm_cmake_config name)