girepository: Add version macros and functions. Fixes #200
This adds the following macros and functions: GI_MAJOR_VERSION, GI_MICRO_VERSION, GI_MINOR_VERSION, GI_CHECK_VERSION, gi_get_major_version,gi_get_micro_version, gi_get_minor_version. Since we share a prefix with glib we have to namespace these by using the gi_ prefix. g_gi would also work but we already export symbols with gi_ like gi_cclosure_marshal_generic(), gi_type_tag_get_ffi_type() and gi_type_info_extract_ffi_return_value(), so let's not add another naming scheme.
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