add a default value for CMAKE_BUILD_TYPE, resolves error when it is not set

CMakeLists.txt

@@ -20,6 +20,13 @@ set (OPENMW_VERSION_RELEASE 0)
 
 set (OPENMW_VERSION "${OPENMW_VERSION_MAJOR}.${OPENMW_VERSION_MINOR}.${OPENMW_VERSION_RELEASE}")
 
+# Add a sensible build type default and warning because empty means no optimization and no debug info.
+if(NOT CMAKE_BUILD_TYPE)
+    message("WARNING: CMAKE_BUILD_TYPE is not defined!\n         Defaulting to CMAKE_BUILD_TYPE=RelWithDebInfo. Use ccmake to set a proper value.")
+    set(CMAKE_BUILD_TYPE RelWithDebInfo
+    CACHE STRING "Type of build, options are: None Debug Release RelWithDebInfo MinSizeRel." FORCE)
+endif(NOT CMAKE_BUILD_TYPE)
+
 # Debug suffix for plugins
 set(DEBUG_SUFFIX "")
 if (${CMAKE_BUILD_TYPE} STREQUAL "Debug")