remove strnlen usage

Under Cygwin, it needs _GNU_SOURCE, which is difficult to enable
manually. Usually needs -std=gnu++.

.github/workflows/on_PR_meson.yaml

@@ -145,7 +145,7 @@ jobs:
             pkgconf
       - name: Compile and Test
         run: |
-          meson setup build -Dwarning_level=3 -Dcpp_std=gnu++20
+          meson setup build -Dwarning_level=3
           meson compile -C build --verbose
           meson test -C build --verbose
   MacOS:

src/bmffimage.cpp

@@ -363,7 +363,7 @@ uint64_t BmffImage::boxHandler(std::ostream& out /* = std::cout*/, Exiv2::PrintS
       // Check that the string has a '\0' terminator.
       const char* str = data.c_str(skip);
       const size_t maxlen = data.size() - skip;
-      Internal::enforce(maxlen > 0 && strnlen(str, maxlen) < maxlen, Exiv2::ErrorCode::kerCorruptedMetadata);
+      Internal::enforce(maxlen > 0 && std::strlen(str) < maxlen, Exiv2::ErrorCode::kerCorruptedMetadata);
       std::string name(str);
       if (Internal::contains(name, "Exif")) {  // "Exif" or "ExifExif"
         exifID_ = ID;

src/helper_functions.cpp

@@ -13,7 +13,10 @@ std::string string_from_unterminated(const char* data, size_t data_length) {
   if (data_length == 0) {
     return {};
   }
-  const size_t StringLength = strnlen(data, data_length);
+  size_t StringLength = 0;
+  while (StringLength < data_length && data[StringLength] != '\0') {
+    StringLength++;
+  }
   return {data, StringLength};
 }