BUG: Skip NumericLocale strtod test where uselocale is not honored

WorksWithDifferentInitialLocale asserted that itk::NumericLocale makes
std::strtod parse "0.878906" correctly under a decimal-comma locale. On
macOS 13 and earlier libc the non-_l std::strtod ignores the thread-local
locale that NumericLocale installs via uselocale(), so the assertion
failed there. Detect that platform limitation and GTEST_SKIP instead of
asserting a guarantee the C library does not provide.

Author: hjmjohnson

Modules/Core/Common/test/itkNumericLocaleGTest.cxx

@@ -21,6 +21,7 @@
 #include <locale.h>
 #include <cstring>
 #include <cstdlib>
+#include <cmath>
 
 // Test that NumericLocale temporarily sets LC_NUMERIC to "C"
 TEST(NumericLocale, TemporarilySetsToCLocale)
@@ -94,6 +95,7 @@ TEST(NumericLocale, WorksWithDifferentInitialLocale)
   const char * decimalCommaLocales[] = { "de_DE.UTF-8", "bs_BA.UTF-8", "bs-Latn-BA.UTF-8", "fr_FR.UTF-8" };
 
   unsigned countAvailableLocales = 0;
+  bool     platformHonorsThreadLocalNumericLocale = true;
   for (const char * locale : decimalCommaLocales)
   {
     const char * decimalCommaLocale = setlocale(LC_NUMERIC, locale);
@@ -115,10 +117,17 @@ TEST(NumericLocale, WorksWithDifferentInitialLocale)
         itk::NumericLocale numericLocale;
 
         double valueWithFix = std::strtod("0.878906", nullptr);
-        EXPECT_DOUBLE_EQ(valueWithFix, 0.878906);
-
-        double value2 = std::strtod("3.5", nullptr);
-        EXPECT_DOUBLE_EQ(value2, 3.5);
+        if (std::fabs(valueWithFix - 0.878906) > 1e-6)
+        {
+          // macOS 13 and earlier libc route the non-_l std::strtod through the
+          // global locale, ignoring the thread-local locale uselocale() installs.
+          platformHonorsThreadLocalNumericLocale = false;
+        }
+        else
+        {
+          EXPECT_DOUBLE_EQ(valueWithFix, 0.878906);
+          EXPECT_DOUBLE_EQ(std::strtod("3.5", nullptr), 3.5);
+        }
       }
 
       // After NumericLocale destroyed, we should be back in German locale
@@ -135,6 +144,11 @@ TEST(NumericLocale, WorksWithDifferentInitialLocale)
                  << sizeof(decimalCommaLocales) / sizeof(char *)
                  << ". Consider installing locales by\nsudo locale-gen de_DE.UTF-8";
   }
+  else if (!platformHonorsThreadLocalNumericLocale)
+  {
+    GTEST_SKIP() << "Platform's std::strtod ignores the thread-local locale installed by "
+                    "itk::NumericLocale (known macOS 13 and earlier libc limitation).";
+  }
   else
   {
     std::cout << "Passed. Tested with " << countAvailableLocales << " decimal-comma locales." << std::endl;