[PATCH] Rework Reporting Of Config Parse Errors To Use Callback

This patch ldo_config_parse-20190729.patch reworks the FcConfigMessage routine in fcxml.c to pass all messages through a callback action. The idea is that it becomes easy to add alternate versions of the FcConfigParseAndLoadxxx routines that specify a caller-specified callback for capturing the messages, rather than sending them direct to stderr.

Initially I had the callback details stored as fields in the _FcConfigParse structure. Then I discovered that messages could be reported in situations where such a structure did not exist. So I put them in a separate _MessageCallback structure. So this structure has to be passed as a separate arg to every FcConfigMessage call.