src/libcxxabi-20.1.2.src のみに存在: CYGWIN-PATCHES
diff -ru origsrc/libcxxabi-20.1.2.src/include/__cxxabi_config.h src/libcxxabi-20.1.2.src/include/__cxxabi_config.h
--- origsrc/libcxxabi-20.1.2.src/include/__cxxabi_config.h	2025-04-02 08:44:12.000000000 +0900
+++ src/libcxxabi-20.1.2.src/include/__cxxabi_config.h	2025-04-19 20:43:12.695020700 +0900
@@ -31,7 +31,7 @@
 #  define _LIBCXXABI_COMPILER_IBM
 #endif
 
-#if defined(_WIN32)
+#if defined(_WIN32) || defined(__CYGWIN__)
  #if defined(_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS) || (defined(__MINGW32__) && !defined(_LIBCXXABI_BUILDING_LIBRARY))
   #define _LIBCXXABI_HIDDEN
   #define _LIBCXXABI_DATA_VIS
@@ -66,7 +66,7 @@
  #endif
 #endif
 
-#if defined(_LIBCXXABI_COMPILER_MSVC)
+#if defined(_LIBCXXABI_COMPILER_MSVC) || defined(__CYGWIN__)
 #define _LIBCXXABI_WEAK
 #else
 #define _LIBCXXABI_WEAK __attribute__((__weak__))
diff -ru origsrc/libcxxabi-20.1.2.src/include/cxxabi.h src/libcxxabi-20.1.2.src/include/cxxabi.h
--- origsrc/libcxxabi-20.1.2.src/include/cxxabi.h	2025-04-02 08:44:12.000000000 +0900
+++ src/libcxxabi-20.1.2.src/include/cxxabi.h	2025-04-19 20:41:50.667967500 +0900
@@ -26,7 +26,7 @@
 #ifdef __cplusplus
 
 namespace std {
-#if defined(_WIN32)
+#if defined(_WIN32) || defined(__CYGWIN__)
 class _LIBCXXABI_TYPE_VIS type_info; // forward declaration
 #else
 class type_info; // forward declaration