[libunwind] Emit dependent libraries only when detected by CMake

[mgorny]

Summary

996e62eef75 added Linux-specific dependent libraries to libunwind
sources. As a result, building libunwind with modern LLD on *BSD
started failing due to trying to link libdl. Instead, add those
libraries only if they were detected by CMake.

While technically we could create a long list of systems that need -ldl
and -lpthread, maintaining a duplicate list makes little sense when
CMake needs to detect it for non-LLD systems anyway. Remove existing
system exceptions since they should be covered by the CMake check
anyway.

Remove -D_LIBUNWIND_HAS_COMMENT_LIB_PRAGMA since it is no longer
explicitly needed, if we make the library-specific defines dependent
on presence of this pragma support.

[phosek]

LGTM, thanks! Would it be possible to make the same change to libc++abi and libc++ as well (since they also use dependent library feature)?

[mgorny]

I'll look into it a bit later. I've focused on libunwind since it's breaking NetBSD buildbot.

libunwind/CMakeLists.txt

 endif()
 
 # Disable DLL annotations on Windows for static builds.
 if (WIN32 AND LIBUNWIND_ENABLE_STATIC AND NOT LIBUNWIND_ENABLE_SHARED)
   add_definitions(-D_LIBUNWIND_DISABLE_VISIBILITY_ANNOTATIONS)
 endif()
 
 if (LIBUNWIND_HAS_COMMENT_LIB_PRAGMA)
-  add_definitions(-D_LIBUNWIND_HAS_COMMENT_LIB_PRAGMA)
+  if (LIBUNWIND_HAS_DL_LIB)
+    add_definitions(-D_LIBUNWIND_LINK_DL_LIB)
+  endif()
+  if (LIBUNWIND_HAS_PTHREAD_LIB)
+    add_definitions(-D_LIBUNWIND_LINK_PTHREAD_LIB)
+  endif()
 endif()
 
 #===============================================================================
 # Setup Source Code
 #===============================================================================
 
 include_directories(include)
 

libunwind/src/AddressSpace.hpp

#if !defined(_LIBUNWIND_IS_BAREMETAL) && !defined(_WIN32)
     #define _LIBUNWIND_USE_DLADDR 1
   #else
     #define _LIBUNWIND_USE_DLADDR 0
   #endif
 #endif
 
 #if _LIBUNWIND_USE_DLADDR
 #include <dlfcn.h>
-#if defined(__unix__) && defined(__ELF__) && defined(_LIBUNWIND_HAS_COMMENT_LIB_PRAGMA)
+#if defined(__ELF__) && defined(_LIBUNWIND_LINK_DL_LIB)
 #pragma comment(lib, "dl")
 #endif
 #endif
 
 #if defined(_LIBUNWIND_ARM_EHABI)
 struct EHABIIndexEntry {
   uint32_t functionOffset;
   uint32_t data;

libunwind/src/RWMutex.hpp

 
 #ifndef __RWMUTEX_HPP__
 #define __RWMUTEX_HPP__
 
 #if defined(_WIN32)
 #include <windows.h>
 #elif !defined(_LIBUNWIND_HAS_NO_THREADS)
 #include <pthread.h>
-#if defined(__unix__) && !defined(__ANDROID__) && defined(__ELF__) && defined(_LIBUNWIND_HAS_COMMENT_LIB_PRAGMA)
+#if defined(__ELF__) && defined(_LIBUNWIND_LINK_PTHREAD_LIB)
 #pragma comment(lib, "pthread")
 #endif
 #endif
 
 namespace libunwind {
 
 #if defined(_LIBUNWIND_HAS_NO_THREADS)