The "generic" locale code in GCC's libstdc++ used on openindiana
throws an exception if it notices a non-"C" locale component; avoid
crashes by forcing use of the "C" locale for everything.

Disable several test cases which set non-C locales

--- gnucash-5.11/gnucash/gnucash.cpp.~1~	Mon Nov 11 18:02:45 2024
+++ gnucash-5.11/gnucash/gnucash.cpp	Thu May  1 07:47:54 2025
@@ -285,6 +285,7 @@
 int
 main(int argc, char ** argv)
 {
+    setenv("LC_ALL", "C", 1);
     Gnucash::Gnucash application (PROJECT_NAME);
 #ifdef __MINGW32__
     boost::nowide::args a(argc, argv); // Fix arguments - make them UTF-8
--- gnucash-5.11/libgnucash/engine/test/test-gnc-date.c.~1~	Mon Nov 11 18:02:45 2024
+++ gnucash-5.11/libgnucash/engine/test/test-gnc-date.c	Thu May  1 12:18:36 2025
@@ -852,6 +852,7 @@
                      ==, strlen (buff));
     g_assert_cmpstr (buff, ==, "2045-06-16");
 
+#if 0
     qof_date_format_set (QOF_DATE_FORMAT_LOCALE);
     test_gnc_setlocale (LC_TIME, "en_US");
     memset ((gpointer)buff, 0, sizeof (buff));
@@ -901,6 +902,9 @@
                      ==, strlen (buff));
     strftime(ans, MAX_DATE_LENGTH, GNC_D_FMT, &tm3);
     g_assert_cmpstr (buff, ==, ans);
+#else
+    (void)ans;
+#endif
 
     setlocale (LC_TIME, locale);
     g_free (locale);
--- gnucash-5.11/libgnucash/engine/test/gtest-gnc-numeric.cpp.~1~	Sat Mar 29 12:02:57 2025
+++ gnucash-5.11/libgnucash/engine/test/gtest-gnc-numeric.cpp	Thu May  1 12:24:46 2025
@@ -289,6 +289,7 @@
     GncNumeric rational_string(123, 456);
     output << rational_string;
     EXPECT_EQ("123/456", output.str());
+#if 0
     try
     {
         auto loc = std::locale("fr_FR.utf8");
@@ -320,6 +321,7 @@
     output.str("");
     output << rational_string;
     EXPECT_EQ("123/456", output.str());
+#endif
 }
 
 TEST(gncnumeric_stream, input_stream)