[LLVM][Cygwin] Remove special case for CXX extensions on Cygwin. (llvm#138328)

This is no longer necessary, and results in an inconvenient define of
`i386` on i386 Cygwin targets which breaks compiling
llvm/include/llvm/ExecutionEngine/JITLink/i386.h.

Signed-off-by: Jeremy Drake <[email protected]>

Author: jeremyd2019

llvm/CMakeLists.txt

@@ -77,14 +77,7 @@ endif()
 
 set(CMAKE_CXX_STANDARD ${LLVM_REQUIRED_CXX_STANDARD} CACHE STRING "C++ standard to conform to")
 set(CMAKE_CXX_STANDARD_REQUIRED YES)
-
-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()
+set(CMAKE_CXX_EXTENSIONS NO)
 
 if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
   message(FATAL_ERROR "