[libunwind] Emit dependent libraries only when detected by CMake

996e62e 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.

Differential Revision: https://reviews.llvm.org/D70868

libunwind/CMakeLists.txt

@@ -352,7 +352,12 @@ if (WIN32 AND LIBUNWIND_ENABLE_STATIC AND NOT LIBUNWIND_ENABLE_SHARED)
 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()
 
 #===============================================================================

libunwind/src/AddressSpace.hpp

@@ -27,7 +27,7 @@
 
 #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

libunwind/src/RWMutex.hpp

@@ -17,7 +17,7 @@
 #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