Provide means to change on error behaviour

[namaenonaimumei]

Provide C-style callback function to call on error.

• default behaviour left unchanged (stream to stderr)
• expose logging output to end-user with set_error_callback()

Added this as ftgl-utils source files, since wasn't sure where else to place it, as there isn't any existing common header shared in codebase.

References rougier/freetype-gl#241

[namaenonaimumei]

Not sure what's causing AppVeyor linkage to fail here, since there shouldn't be any breaking changes.

Was testing with MSVS build and forgot to update CMakeList - should be fine now.

[rougier]

Thanks.