Remove --enable-strict configure option.

GLib/GTK+ uses different deprecation macros nowadays.

The MIN_REQUIRED macros do a better job of catching deprecated symbol
usage up to the minimum build requirement, and the MAX_ALLOWED macros
catch symbol usage that violates our minimum build requirement.

These newer deprecation macros are always enabled, so we don't need a
special configure flag for them.

(cherry picked from commit c6e8e6aaac1181a9a7c932365ebe3b18c67467fa)