debug tests Makefile: don't use -pthread when on Windows

Currently, I get the following error:

g++ -gdwarf-2 -pthread -o ../bin/MultiThreadRunControl.exe
MultiThreadRunControl.cc
c:/mingw/bin/../lib/gcc/mingw32/4.8.1/../../../../mingw32/bin/ld.exe:
cannot find -lpthread

I could install the pthreads package for mingw, and it would probably
work, but we don't use pthreads on windows, so it's better to just not
link with it.

Change-Id: I5deb58c5b69a98b77e9e9a4a744c6815c830cf20
Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Signed-off-by: Marc Khouzam <marc.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/37611