fix: change the find order in PyCodec.

- now PyCodec will try to use Iconv first.
- re-claim the meaning of YYCC_FEAT_ICONV macro.

src/yycc/encoding/iconv.cpp

@@ -1,6 +1,6 @@
 #include "iconv.hpp"
 
-#if YYCC_FEAT_ICONV || !defined(YYCC_OS_WINDOWS)
+#if defined(YYCC_FEAT_ICONV)
 
 #include "../macro/endian_detector.hpp"
 #include <cerrno>

src/yycc/encoding/iconv.hpp

@@ -48,7 +48,7 @@ namespace yycc::encoding::iconv {
     template<typename T>
     using ConvResult = std::expected<T, ConvError>;
 
-#if YYCC_FEAT_ICONV || !defined(YYCC_OS_WINDOWS)
+#if defined(YYCC_FEAT_ICONV)
 
     /// @brief Char -> UTF8
     class CharToUtf8 {

src/yycc/encoding/pycodec.hpp

@@ -7,15 +7,18 @@
 #include <expected>
 
 // Choose the backend of PyCodec module
-#if defined(YYCC_OS_WINDOWS) && defined(YYCC_STL_MSSTL)
-#include "windows.hpp"
-#define YYCC_PYCODEC_WIN32_BACKEND
-#define YYCC_PYCODEC_BACKEND_NS ::yycc::encoding::windows
-#elif YYCC_FEAT_ICONV || !defined(YYCC_OS_WINDOWS)
+#if defined(YYCC_FEAT_ICONV)
+// We try Iconv first in any cases.
 #include "iconv.hpp"
 #define YYCC_PYCODEC_ICONV_BACKEND
 #define YYCC_PYCODEC_BACKEND_NS ::yycc::encoding::iconv
+#elif defined(YYCC_OS_WINDOWS) && defined(YYCC_STL_MSSTL)
+// If we can not use Iconv, we try to fallback to Windows implementation.
+#include "windows.hpp"
+#define YYCC_PYCODEC_WIN32_BACKEND
+#define YYCC_PYCODEC_BACKEND_NS ::yycc::encoding::windows
 #else
+// No viable implementation.
 #error "Can not find viable encoding convertion solution in current environment for PyCodec module."
 #endif