[CMake] Fix the -nodefaultlibs check.

[cdavis5x]

Summary

This is a linker flag, so we want to pass it in CMAKE_REQUIRED_FLAGS.
That way, it'll get passed during the link step.

Add some libraries on MinGW and Darwin, which need them to successfully
link.

[cdavis5x]

Ping.

[mstorsjo]

That way, it'll get passed during the link step.

When I do shared builds of libunwind (for mingw), I do get -nodefaultlibs passed, so I'm a little puzzled about why that happens. Aside from that, this change looks sane to me, and doesn't break my build.

[rnk]

cmake/config-ix.cmake

43

Do we really need a dynamic configure check here? Where is nodefaultlibs unsupported, just MSVC? If so, I'd rather drop the feature check and just say if (NOT MSVC). Does libunwind even build with MSVC?

cmake/config-ix.cmake

 
+include(CMakePushCheckState)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 include(CheckLibraryExists)
 
 check_library_exists(c fopen "" LIBUNWIND_HAS_C_LIB)
 
 if (NOT LIBUNWIND_USE_COMPILER_RT)
   check_library_exists(gcc_s __gcc_personality_v0 "" LIBUNWIND_HAS_GCC_S_LIB)
   check_library_exists(gcc __absvdi2 "" LIBUNWIND_HAS_GCC_LIB)
 endif()
 
+if (LIBUNWIND_USE_COMPILER_RT)
+  find_compiler_rt_library(builtins LIBUNWIND_BUILTINS_LIBRARY)
+elseif(LIBUNWIND_HAS_GCC_S_LIB)
+  set(LIBUNWIND_BUILTINS_LIBRARY gcc_s)
+elseif(LIBUNWIND_HAS_GCC_LIB)
+  set(LIBUNWIND_BUILTINS_LIBRARY gcc)
+endif()
+
+function(check_linker_flag flag out_var)
+  cmake_push_check_state()
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} ${flag}")
+  if(MINGW)
+    set(CMAKE_REQUIRED_LIBRARIES mingw32 mingwex moldname msvcrt kernel32 "${LIBUNWIND_BUILTINS_LIBRARY}")
+  elseif(APPLE)
+    set(CMAKE_REQUIRED_LIBRARIES System)
+  elseif(LIBUNWIND_HAS_C_LIB)
+    set(CMAKE_REQUIRED_LIBRARIES c)
+  endif()
+  check_c_compiler_flag("" ${out_var})
+  cmake_pop_check_state()
+endfunction()
+
 # libunwind is built with -nodefaultlibs, so we want all our checks to also
 # use this option, otherwise we may end up with an inconsistency between
 # the flags we think we require during configuration (if the checks are
 # performed without -nodefaultlibs) and the flags that are actually
 # required during compilation (which has the -nodefaultlibs). libc is
 # required for the link to go through. We remove sanitizers from the
 # configuration checks to avoid spurious link errors.
-check_c_compiler_flag(-nodefaultlibs LIBUNWIND_HAS_NODEFAULTLIBS_FLAG)
+check_linker_flag(-nodefaultlibs LIBUNWIND_HAS_NODEFAULTLIBS_FLAG)
 if (LIBUNWIND_HAS_NODEFAULTLIBS_FLAG)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
-  if (LIBUNWIND_HAS_C_LIB)
+  if (APPLE)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES System)
+  elseif (LIBUNWIND_HAS_C_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()
   if (LIBUNWIND_USE_COMPILER_RT)
-    find_compiler_rt_library(builtins LIBUNWIND_BUILTINS_LIBRARY)
     list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBUNWIND_BUILTINS_LIBRARY}")
   else ()
     if (LIBUNWIND_HAS_GCC_S_LIB)
       list(APPEND CMAKE_REQUIRED_LIBRARIES gcc_s)
     endif ()
     if (LIBUNWIND_HAS_GCC_LIB)
       list(APPEND CMAKE_REQUIRED_LIBRARIES gcc)
     endif ()