This reverts commits:
• 9ddcc79502349e02a1e796b363d891e177337af5
• ae02adc3c3795bb32348c07d8f48504b41e9e2c7
g_date_time_format() supports a few non-standard format placeholders:
• %:z
• %::z
• %:::z
These are all gnulib strtime() extensions, and hence are not recognised
by the compiler when the function is annotated with G_GNUC_STRFTIME.
However, this wasn’t noticed when we originally merged this change
because the errors were disabled in the tests which covered those
placeholders.