repair cygwin build

[vtjnash]

Summary

This is needed for cross-compiling LLVM from Cygwin, but it had gotten
deleted in rG2724d9e12960cc1d93eeabbfc9aa1bffffa041cc

[compnerd]

That really is unfortunate. LLVM generally does a pretty good job of sticking to standard C++. The symbols you claim are missing aren't C++ symbols but rather C symbols. I suppose that this doesn't hurt the other targets.

[vtjnash]

I think with the move to newer C++ standards, some of the missing functionality in C could be replicated (or use the _WIN32 code paths for __CYGWIN__ in more places). I tried that for a bit, but it was pieces like struct sigaction, realpath, and getsid missing, which are very platform-specific.

Thanks.

llvm/CMakeLists.txt

 endif()
 
 project(LLVM
   VERSION ${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}
   LANGUAGES C CXX ASM)
 
 set(CMAKE_CXX_STANDARD 14 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()
 
 if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
   message(STATUS "No build type selected, default to Debug")
   set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)" FORCE)
 endif()
 
 # Side-by-side subprojects layout: automatically set the
 # LLVM_EXTERNAL_${project}_SOURCE_DIR using LLVM_ALL_PROJECTS