utils: Use nominative/genitive month names correctly

These changes are needed since the standalone (nominative) vs. full date
format (genitive) month names support has been introduced in Linux with
glibc 2.27.  They are also supported in *BSD systems.

See also:
https://bugzilla.gnome.org/show_bug.cgi?id=749206
https://sourceware.org/bugzilla/show_bug.cgi?id=10871

https://gitlab.gnome.org/GNOME/gnome-calendar/issues/125

meson.build

@@ -78,6 +78,30 @@ nl_time_first_weekday_src = '''
 config_h.set('HAVE__NL_TIME_FIRST_WEEKDAY', cc.compiles(nl_time_first_weekday_src),
          description: 'Define if _NL_TIME_FIRST_WEEKDAY is available')
 
+# Not all systems support nl_langinfo (ALTMON_*)
+altmon_src = '''
+  /* _GNU_SOURCE is required by glibc 2.27.  */
+  #define _GNU_SOURCE
+  #include <langinfo.h>
+  int main() {
+    nl_langinfo (ALTMON_1);
+    nl_langinfo (ALTMON_2);
+    nl_langinfo (ALTMON_3);
+    nl_langinfo (ALTMON_4);
+    nl_langinfo (ALTMON_5);
+    nl_langinfo (ALTMON_6);
+    nl_langinfo (ALTMON_7);
+    nl_langinfo (ALTMON_8);
+    nl_langinfo (ALTMON_9);
+    nl_langinfo (ALTMON_10);
+    nl_langinfo (ALTMON_11);
+    nl_langinfo (ALTMON_12);
+  };
+'''
+
+config_h.set('HAVE_ALTMON', cc.compiles(altmon_src),
+         description: 'Define if ALTMON_* constants are available')
+
 # Compiler flags
 common_flags = [
   '-DHAVE_CONFIG_H',

src/gcal-date-chooser.c

@@ -224,20 +224,6 @@ calendar_get_weekday_name (gint i)
   return text;
 }
 
-/* 0 = january */
-static gchar *
-calendar_get_month_name (gint i)
-{
-  GDateTime *date;
-  gchar *text;
-
-  date = g_date_time_new_local (2015, i + 1, 1, 1, 1, 1);
-  text = g_date_time_format (date, "%B");
-  g_date_time_unref (date);
-
-  return text;
-}
-
 static void
 calendar_init_weekday_display (GcalDateChooser *self)
 {
@@ -257,7 +243,7 @@ format_month (GcalMultiChoice *choice,
               gint             value,
               gpointer         data)
 {
-  return calendar_get_month_name (value);
+  return g_strdup (gcal_get_month_name (value));
 }
 
 static void
@@ -269,9 +255,8 @@ calendar_init_month_display (GcalDateChooser *self)
 
   for (i = 0; i < 12; i++)
     {
-      month = calendar_get_month_name (i);
-      months[i] = g_strdup_printf ("%s", month);
-      g_free (month);
+      month = gcal_get_month_name (i);
+      months[i] = g_strdup (month);
     }
 
   months[12] = NULL;

src/gcal-utils.c

@@ -20,6 +20,9 @@
 
 #define G_LOG_DOMAIN "Utils"
 
+/* langinfo.h in glibc 2.27 defines ALTMON_* only if _GNU_SOURCE is defined.  */
+#define _GNU_SOURCE
+
 #include "gcal-enums.h"
 #include "gcal-utils.h"
 #include "gcal-event-widget.h"
@@ -57,6 +60,29 @@ ab_day[7] =
 static const gint
 month_item[12] =
 {
+  /* ALTMON_* constants have been introduced in glibc 2.27 (Feb 1, 2018), also
+   * have been supported in *BSD family (but not in OS X) since 1990s.
+   * If they exist they are the correct way to obtain the month names in
+   * nominative case, standalone, without the day number, as used in the
+   * calendar header.  This is obligatory in some languages (Slavic, Baltic,
+   * Greek, etc.) but also recommended to use in all languages because for
+   * other languages there is no difference between ALTMON_* and MON_*.
+   * If ALTMON_* is not supported then we must use MON_*.
+   */
+#ifdef HAVE_ALTMON
+  ALTMON_1,
+  ALTMON_2,
+  ALTMON_3,
+  ALTMON_4,
+  ALTMON_5,
+  ALTMON_6,
+  ALTMON_7,
+  ALTMON_8,
+  ALTMON_9,
+  ALTMON_10,
+  ALTMON_11,
+  ALTMON_12
+#else
   MON_1,
   MON_2,
   MON_3,
@@ -69,6 +95,7 @@ month_item[12] =
   MON_10,
   MON_11,
   MON_12
+#endif
 };
 
 #define SCROLL_HARDNESS 10.0