Adjust for backtrace(3) on BSDs

In file included from offline_compiler/utilities/linux/safety_caller_linux.cpp:10:
offline_compiler/utilities/linux/safety_guard_linux.h:36:25: error: implicit conversion loses integer precision: 'size_t' (aka 'unsigned long') to 'int' [-Werror,-Wshorten-64-to-32]
        backtraceSize = backtrace(addresses, callstackDepth);
                      ~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

ld: error: undefined symbol: backtrace
>>> referenced by safety_caller_linux.cpp
>>>               CMakeFiles/ocloc.dir/utilities/linux/safety_caller_linux.cpp.o:(SafetyGuardLinux::sigAction(int, __siginfo*, void*))
ld: error: undefined symbol: backtrace_symbols
>>> referenced by safety_caller_linux.cpp
>>>               CMakeFiles/ocloc.dir/utilities/linux/safety_caller_linux.cpp.o:(SafetyGuardLinux::sigAction(int, __siginfo*, void*))

--- offline_compiler/utilities/linux/safety_guard_linux.h.orig	2019-11-22 15:41:47 UTC
+++ offline_compiler/utilities/linux/safety_guard_linux.h
@@ -33,7 +33,7 @@ class SafetyGuardLinux {
         char **callstack;
         int backtraceSize = 0;
 
-        backtraceSize = backtrace(addresses, callstackDepth);
+        backtraceSize = (int)backtrace(addresses, callstackDepth);
         callstack = backtrace_symbols(addresses, backtraceSize);
 
         for (int i = 0; i < backtraceSize; ++i) {
--- offline_compiler/CMakeLists.txt.orig	2019-11-29 14:23:34 UTC
+++ offline_compiler/CMakeLists.txt
@@ -124,6 +124,9 @@ endif()
 
 if(UNIX)
   target_link_libraries(ocloc dl pthread)
+  if(CMAKE_SYSTEM_NAME MATCHES "DragonFly|FreeBSD|NetBSD|OpenBSD")
+    target_link_libraries(ocloc execinfo)
+  endif()
 endif()
 
 target_link_libraries(ocloc elflib)