Correct abs function should be used (clang -Wabsolute-value)
introspection: Add default functions returning NULL

... and add gboolean module->has_introspection
Clang: -Wabsolute-value is avaliable only in trunk
introspection: Add type name to .header

This is mostly useful for structs, enums, unions
Merge pull request #502 from LebedevRI/cmake-werror-aka-guidelines

Correct abs function should be used (clang -Wabsolute-value)