diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -52,7 +52,13 @@
 
 set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform to")
 set(CMAKE_CXX_STANDARD_REQUIRED YES)
-set(CMAKE_CXX_EXTENSIONS NO)
+if (CYGWIN)
+  # Cygwin is a bit stricter and lack things like 'strdup', 'stricmp', etc in
+  # c++xx mode.
+  set(CMAKE_CXX_EXTENSIONS YES)
+else()
+  set(CMAKE_CXX_EXTENSIONS NO)
+endif()
 
 if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
   message(STATUS "No build type selected, default to Debug")