libuv: Explicitly enable GNU APIs on Cygwin

Backport upstream libuv commit `17ea56ee73` build: Improve build by
cmake for Cygwin, 2023-01-20, v1.45.0~68) to define `_GNU_SOURCE`.

Utilities/cmlibuv/CMakeLists.txt

@@ -165,6 +165,7 @@ if(CMAKE_SYSTEM_NAME MATCHES "CYGWIN" OR CMAKE_SYSTEM_NAME MATCHES "MSYS")
     include/uv/posix.h
     )
   list(APPEND uv_defines
+    _GNU_SOURCE
     )
   list(APPEND uv_sources
     src/unix/cygwin.c