[gnc-datetime] add boost UK/US/ISO date parsers

Thus "30 Sep 2024" and "30 September 2024" are accepted.
But ICU's "30 Sept 2024" is rightfully rejected.

libgnucash/engine/gnc-datetime.cpp

@@ -84,6 +84,10 @@ static constexpr auto ticks_per_second = INT64_C(1000000000);
  * without separators.
  */
 const std::vector<GncDateFormat> GncDate::c_formats ({
+    GncDateFormat { N_("UK date"), boost::gregorian::from_uk_string },
+    GncDateFormat { N_("US date"), boost::gregorian::from_us_string },
+    GncDateFormat { N_("ISO delimited date"), boost::gregorian::from_string },
+    GncDateFormat { N_("ISO date"), boost::gregorian::from_undelimited_string },
     GncDateFormat {
         N_("y-m-d"),
         "(?:"                                   // either y-m-d
@@ -617,6 +621,12 @@ GncDateImpl::GncDateImpl(const std::string str, const std::string fmt) :
     if (iter == GncDate::c_formats.cend())
         throw std::invalid_argument(N_("Unknown date format specifier passed as argument."));
 
+    if (iter->m_str_to_date)
+    {
+        m_greg = (*iter->m_str_to_date)(str);
+        return;
+    }
+
     boost::regex r(iter->m_re);
     boost::smatch what;
     if(!boost::regex_search(str, what, r))  // regex didn't find a match

libgnucash/engine/gnc-datetime.hpp

@@ -29,6 +29,10 @@
 #include <memory>
 #include <string>
 #include <vector>
+#include <functional>
+#include <optional>
+
+#include <boost/date_time/gregorian/gregorian.hpp>
 
 typedef struct
 {
@@ -172,6 +176,8 @@ class GncDateTime
  * GncDate::c_formats class variable and work with those.
  */
 
+using StringToDate = std::function<boost::gregorian::date(const std::string&)>;
+
 class GncDateFormat
 {
 public:
@@ -182,13 +188,16 @@ class GncDateFormat
      */
     GncDateFormat (const char* fmt, const char* re) :
     m_fmt(fmt), m_re(re) {}
+    GncDateFormat (const char* fmt, StringToDate str_to_date) :
+        m_fmt(fmt), m_str_to_date(str_to_date) {}
     /** A string representing the format. */
     const std::string m_fmt;
 private:
     /** Regular expression associated with the format string. This is to and
      * only be used internally by the gnc-datetime code.
      */
     const std::string m_re;
+    std::optional<StringToDate> m_str_to_date;
 
     friend class GncDateImpl;
 };