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.

giversionmacros.h

@@ -153,4 +153,10 @@
 # define GI_AVAILABLE_IN_1_46                 _GI_EXTERN
 #endif
 
+#if defined(GLIB_VERSION_2_60) && GLIB_VERSION_MAX_ALLOWED < GLIB_VERSION_2_60
+# define GI_AVAILABLE_IN_1_60                 GLIB_UNAVAILABLE(2, 60)
+#else
+# define GI_AVAILABLE_IN_1_60                 _GI_EXTERN
+#endif
+
 #endif /* __GIVERSIONMACROS_H__ */