[llvm] [LLVM][Cygwin] Define _GNU_SOURCE on Cygwin as well. (PR #138329)

[via llvm-commits]

jeremyd2019 wrote:

this seems to work
```cmake
--- clang-20.1.4-1.x86_64/origsrc/clang-20.1.4.src/CMakeLists.txt       2025-04-29 16:05:17.000000000 -0700
+++ clang-20.1.4-1.x86_64/src/clang-20.1.4.src/CMakeLists.txt   2025-05-05 13:58:28.443602600 -0700
@@ -68,6 +68,10 @@
   option(CLANG_ENABLE_BOOTSTRAP "Generate the clang bootstrap target" OFF)
   option(LLVM_ENABLE_LIBXML2 "Use libxml2 if available." ON)

+  separate_arguments(LLVM_DEFINITIONS_LIST NATIVE_COMMAND ${LLVM_DEFINITIONS})
+  add_definitions(${LLVM_DEFINITIONS_LIST})
+  list(APPEND CMAKE_REQUIRED_DEFINITIONS ${LLVM_DEFINITIONS_LIST})
+
   include(AddLLVM)
   include(TableGen)
   include(HandleLLVMOptions)
@@ -181,20 +185,19 @@
 check_include_file(sys/resource.h CLANG_HAVE_RLIMITS)

 # This check requires _GNU_SOURCE on linux
+include(CMakePushCheckState)
 check_include_file(dlfcn.h CLANG_HAVE_DLFCN_H)
 if( CLANG_HAVE_DLFCN_H )
   include(CheckLibraryExists)
   include(CheckSymbolExists)
   check_library_exists(dl dlopen "" HAVE_LIBDL)
+  cmake_push_check_state()
   if( HAVE_LIBDL )
     list(APPEND CMAKE_REQUIRED_LIBRARIES dl)
   endif()
-  list(APPEND CMAKE_REQUIRED_DEFINITIONS -D_GNU_SOURCE)
+  #list(APPEND CMAKE_REQUIRED_DEFINITIONS -D_GNU_SOURCE)
   check_symbol_exists(dladdr dlfcn.h CLANG_HAVE_DLADDR)
-  list(REMOVE_ITEM CMAKE_REQUIRED_DEFINITIONS -D_GNU_SOURCE)
-  if( HAVE_LIBDL )
-    list(REMOVE_ITEM CMAKE_REQUIRED_LIBRARIES dl)
-  endif()
+  cmake_pop_check_state()
 endif()

 set(CLANG_RESOURCE_DIR "" CACHE STRING
```

Issues I ran into:
* the `list(REMOVE_ITEM CMAKE_REQUIRED_DEFINITIONS -D_GNU_SOURCE)` wipes out *all* occurences in the list, not just the one it just added.  Better to push/pop (used in other llvm cmake files).  
* HandleLLVMOptions.cmake nukes the `LLVM_DEFINITIONS` variable, so I had to add them *before* including that.

https://github.com/llvm/llvm-project/pull/138329