Index: cmake/config-ix.cmake
===================================================================
--- cmake/config-ix.cmake
+++ cmake/config-ix.cmake
@@ -147,6 +147,9 @@
 
 # function checks
 check_symbol_exists(arc4random "stdlib.h" HAVE_DECL_ARC4RANDOM)
+if(CMAKE_SYSTEM MATCHES "FreeBSD")
+  list(APPEND CMAKE_REQUIRED_LIBRARIES "execinfo")
+endif()
 check_symbol_exists(backtrace "execinfo.h" HAVE_BACKTRACE)
 check_symbol_exists(_Unwind_Backtrace "unwind.h" HAVE__UNWIND_BACKTRACE)
 check_symbol_exists(getpagesize unistd.h HAVE_GETPAGESIZE)
Index: lib/Support/CMakeLists.txt
===================================================================
--- lib/Support/CMakeLists.txt
+++ lib/Support/CMakeLists.txt
@@ -24,6 +24,9 @@
   if( UNIX AND NOT (BEOS OR HAIKU) )
     set(system_libs ${system_libs} m)
   endif()
+  if (HAVE_BACKTRACE AND CMAKE_SYSTEM MATCHES "FreeBSD")
+    set(system_libs ${system_libs} execinfo)
+  endif()
 endif( MSVC OR MINGW )
 
 add_llvm_library(LLVMSupport
Index: lib/Support/Unix/Signals.inc
===================================================================
--- lib/Support/Unix/Signals.inc
+++ lib/Support/Unix/Signals.inc
@@ -45,11 +45,15 @@
 #endif
 #ifdef HAVE__UNWIND_BACKTRACE
 // FIXME: We should be able to use <unwind.h> for any target that has an
-// _Unwind_Backtrace function, but on FreeBSD the configure test passes
-// despite the function not existing, and on Android, <unwind.h> conflicts
+// _Unwind_Backtrace function, but on Android, <unwind.h> conflicts
 // with <link.h>.
 #if defined(__GLIBC__) || defined(__APPLE__)
 #include <unwind.h>
+#elif defined(__FreeBSD__)
+// On FreeBSD we are including unwind.h from libcxxrt which requires
+// _GNU_SOURCE to make _Unwind_Backtrace visible
+#define _GNU_SOURCE
+#include <unwind.h>
 #else
 #undef HAVE__UNWIND_BACKTRACE
 #endif