[clang] [Clang][Cygwin] attempt to fix building shared libclang. (PR #138351)

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https://github.com/jeremyd2019 created https://github.com/llvm/llvm-project/pull/138351

@mstorsjo I don't understand why this isn't working.  We know it works for MinGW right?

>From fa1d7d6dc5316d285ff836991a3e777252c7cf3e Mon Sep 17 00:00:00 2001
From: Jeremy Drake <github at jdrake.com>
Date: Fri, 2 May 2025 14:55:52 -0700
Subject: [PATCH] [Clang][Cygwin] attempt to fix building shared libclang.

---
 clang/include/clang-c/Platform.h    | 2 +-
 clang/tools/libclang/CMakeLists.txt | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/clang/include/clang-c/Platform.h b/clang/include/clang-c/Platform.h
index 67c1fff8ff783..8d341ddd6f8eb 100644
--- a/clang/include/clang-c/Platform.h
+++ b/clang/include/clang-c/Platform.h
@@ -22,7 +22,7 @@ LLVM_CLANG_C_EXTERN_C_BEGIN
 #ifndef CINDEX_NO_EXPORTS
   #define CINDEX_EXPORTS
 #endif
-#ifdef _WIN32
+#if defined(_WIN32) || defined(__CYGWIN__)
   #ifdef CINDEX_EXPORTS
     #ifdef _CINDEX_LIB_
       #define CINDEX_LINKAGE __declspec(dllexport)
diff --git a/clang/tools/libclang/CMakeLists.txt b/clang/tools/libclang/CMakeLists.txt
index 37a939ffcada7..454dfead0bcf5 100644
--- a/clang/tools/libclang/CMakeLists.txt
+++ b/clang/tools/libclang/CMakeLists.txt
@@ -158,7 +158,7 @@ if(ENABLE_STATIC)
 endif()
 
 if(ENABLE_SHARED)
-  if(WIN32)
+  if(WIN32 OR CYGWIN)
     set_target_properties(libclang
       PROPERTIES
       VERSION ${LIBCLANG_LIBRARY_VERSION}