Why does C++ "Hello World" require pthread (which is missing) in Eclipse Mars?

I've been running Eclipse Juno and MinGW 4.6.2 for a long time, but I need full C++11 support so I upgraded to Eclipse Mars and installed MinGW 4.9.3 in a different directory.

In Mars, linking any executable - even in the C++ Hello World project - results in "C:\MinGW_4_9_4\bin...\ld.exe: cannot find -lpthread."

The 4.6.2 directory (for Juno) contains pthread, but the 4.9.3 directory (for Mars) does not.

Why and How is Mars trying to link to pthread? (Can I turn it off?)