Fix SANITIZE=ON for GCC 6

GCC on Ubuntu does not switch to the "gold" linker when the fsanitize
switch is used, so add the relevant linker flags manually:
http://stackoverflow.com/questions/37603238/fsanitize-not-using-gold-linker-in-gcc-6-1

tools/cmake/FindOptions.cmake

@@ -1,6 +1,15 @@
+macro(append var string)
+  set(${var} "${${var}} ${string}")
+endmacro(append)
+
 set(SANITIZE_FLAGS "")
 if(SANITIZE)
   set(SANITIZE_FLAGS "-fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined")
+  if (CMAKE_COMPILER_IS_GNUCC) 
+    # Ubuntu bug for GCC 5.0+ (safe for all versions)
+    append(CMAKE_EXE_LINKER_FLAGS "-fuse-ld=gold")
+    append(CMAKE_SHARED_LINKER_FLAGS "-fuse-ld=gold")
+  endif()
 endif()
 
 set(OPT_FLAGS "-march=native")