currently threading is internal to libvpx so thread safety is unneeded
in libgtest -- visual studio builds already operate in this way as they
do not have pthread.h available by default.

this removes an unconditional link to libpthread using $(extralibs)
should libvpx require it.

Change-Id: I2f278b711f533d0f4d8a6c896833e3e2237d1f45