gdatetime: Add support for %E modifier to g_date_time_format()

The `%E` modifier causes dates to be formatted using an alternative era
representation for years. This doesn’t do anything for most dates, but
in locales such as Thai and Japanese it causes years to be printed using
era names.

In Thai, this means the Thai solar calendar
(https://en.wikipedia.org/wiki/Thai_solar_calendar). In Japanese, this
means Japanese era names
(https://en.wikipedia.org/wiki/Japanese_era_name).

The `%E` modifier syntax follows what’s supported in glibc — see
nl_langinfo(3).

Supporting this is quite involved, as it means loading the `ERA`
description from libc and parsing it.

Unit tests are included.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

Fixes: #3119

glib/gdatetime-private.h

@@ -0,0 +1,85 @@
+/*
+ * Copyright 2023 Philip Withnall
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "glib.h"
+
+G_BEGIN_DECLS
+
+/**
+ * GEraDate:
+ * @type: the type of date
+ * @year: year of the date, in the Gregorian calendar
+ * @month: month of the date, in the Gregorian calendar
+ * @day: day of the date, in the Gregorian calendar
+ *
+ * A date from a #GEraDescriptionSegment.
+ *
+ * If @type is %G_ERA_DATE_SET, @year, @month and @day are valid. Otherwise,
+ * they are undefined.
+ *
+ * Since: 2.80
+ */
+typedef struct {
+  enum {
+    G_ERA_DATE_SET,
+    G_ERA_DATE_PLUS_INFINITY,
+    G_ERA_DATE_MINUS_INFINITY,
+  } type;
+  int year;
+  int month;
+  int day;
+} GEraDate;
+
+int _g_era_date_compare (const GEraDate *date1,
+                         const GEraDate *date2);
+
+/**
+ * GEraDescriptionSegment:
+ * @ref_count: reference count
+ * @direction_multiplier: `-1` or `1` depending on the order of @start_date and
+ *   @end_date
+ * @offset: offset of the first year in the era
+ * @start_date: start date (in the Gregorian calendar) of the era
+ * @end_date: end date (in the Gregorian calendar) of the era
+ * @era_name: (not nullable): name of the era
+ * @era_format: (not nullable): format string to use for `%EY`
+ *
+ * A segment of an `ERA` description string, describing a single era. See
+ * [`nl_langinfo(3)`](man:nl_langinfo(3)).
+ *
+ * Since: 2.80
+ */
+typedef struct {
+  gatomicrefcount ref_count;
+  int direction_multiplier;
+  guint64 offset;
+  GEraDate start_date;  /* inclusive */
+  GEraDate end_date;  /* inclusive */
+  char *era_name;  /* UTF-8 encoded */
+  char *era_format;  /* UTF-8 encoded */
+} GEraDescriptionSegment;
+
+GPtrArray *_g_era_description_parse (const char *desc);
+
+GEraDescriptionSegment *_g_era_description_segment_ref (GEraDescriptionSegment *segment);
+void _g_era_description_segment_unref (GEraDescriptionSegment *segment);
+
+G_END_DECLS