Why am I getting linker errors using -fgnu-tm?

I'm using GCC 6.2.0 and am trying out the experiemental transaction support. Following the manual, I've added "-fgnu-tm" to my cxxflags and added some synchronized {} blocks to my source code. It compiles fine but then I get linker errors like below:

MyFile.cc:2865: undefined reference to `_ITM_beginTransaction'
MyFile.cc:2873: undefined reference to `_ITM_commitTransaction'

I tried adding "-fgnu-tm" to linker flags as well but it didn't solve the problem. Why am I getting these errors and what can I do to fix it?