[libunwind] [cmake] Add MINGW_LIBRARIES to the linker flags

[mstorsjo]

Summary

This is essential when building with -nodefaultlibs.

In some CMake versions (noticed in 3.5.1), the same libraries are picked up from CMAKE_REQUIRED_LIBRARIES in some exceptional situations (if CXX probing failed, due to libc++ not being built yet, the libraries from CMAKE_REQUIRED_LIBRARIES are used for linking the target library), but not at all in other newer CMake versions (3.10).

This is similar to what already is done in libcxxabi in SVN r302760 and libcxx in SVN r312498.

[phosek]

LGTM

libunwind/trunk/src/CMakeLists.txt

if (LIBUNWIND_ENABLE_THREADS)
   append_if(libraries LIBUNWIND_HAS_PTHREAD_LIB pthread)
 endif()
 
 # Setup flags.
 append_if(LIBUNWIND_CXX_FLAGS LIBUNWIND_HAS_NO_RTTI_FLAG -fno-rtti)
 
 append_if(LIBUNWIND_LINK_FLAGS LIBUNWIND_HAS_NODEFAULTLIBS_FLAG -nodefaultlibs)
 
+# MINGW_LIBRARIES is defined in config-ix.cmake
+append_if(libraries MINGW "${MINGW_LIBRARIES}")
+
 if (LIBUNWIND_HAS_NO_EXCEPTIONS_FLAG AND LIBUNWIND_HAS_FUNWIND_TABLES)
   list(APPEND LIBUNWIND_COMPILE_FLAGS -fno-exceptions)
   list(APPEND LIBUNWIND_COMPILE_FLAGS -funwind-tables)
 elseif (LIBUNWIND_ENABLE_SHARED)
   message(FATAL_ERROR
           "Compiler doesn't support generation of unwind tables if exception "
           "support is disabled.  Building libunwind DSO with runtime dependency "
           "on C++ ABI library is not supported.")