[gnc-date][API] find locale's start of week using ICU.

gnc_start_of_week

* ICU has a mature C++ api, so prefer that one in our C++ code
* Use PERR instead of fprintf for consistent reporting
* Add the ICU specific linker flags to the test case

libgnucash/engine/gnc-date.cpp

@@ -48,6 +48,7 @@ extern "C"
 }
 
 #include <cinttypes>
+#include <unicode/calendar.h>
 
 #include "gnc-date.h"
 #include "gnc-date-p.h"
@@ -201,6 +202,30 @@ gnc_gmtime (const time64 *secs)
 
 }
 
+gint
+gnc_start_of_week (void)
+{
+    /* icu's day of week is 1 based. Using 0 here to mean unset or error while setting */
+    static int cached_result = 0;
+
+    if (!cached_result)
+    {
+        UErrorCode err = U_ZERO_ERROR;
+        auto cal = icu::Calendar::createInstance (err);
+        if (!cal)
+        {
+            PERR("ICU error: %s\n", u_errorName (err));
+            return 0;
+        }
+
+        /* 1 for sunday, 2 for monday, etc. */
+        cached_result = cal->getFirstDayOfWeek (err);
+        delete cal;
+    }
+
+    return cached_result;
+}
+
 time64
 gnc_mktime (struct tm* time)
 {

libgnucash/engine/gnc-date.h

@@ -185,6 +185,11 @@ struct tm* gnc_localtime_r (const time64 *secs, struct tm* time);
  */
 struct tm* gnc_gmtime (const time64 *secs);
 
+/** \brief returns an integer corresponding to locale start of week
+ *  \return An integer 1=Sunday, 2=Monday etc. If error, return 0.
+ */
+gint gnc_start_of_week (void);
+
 /** \brief calculate seconds from the epoch given a time struct
  *  \param time: A struct tm* containing the date-time information.
  *  The time is understood to be in the current local time zone.

libgnucash/engine/test/CMakeLists.txt

@@ -99,6 +99,7 @@ set(gtest_qof_LIBS
   ${GOBJECT_LDFLAGS}
   ${GMODULE_LDFLAGS}
   ${GTHREAD_LDFLAGS}
+  ${ICU4C_I18N_LDFLAGS}
   ${Boost_LIBRARIES}
   ${GTEST_LIB})