Remove compiler warnings
Explicitly request 2 threads when testing slice threading

Previously the default value of iMultipleThreadIdc was 0,
which made sure threads were used for the test with multiple
slices (if run on a multicore machine). Now the default is 1,
so multiple threads has to be requested.

Explicitly request 2 threads, to make sure the threading code is
tested, even on machines with only one core.
Merge pull request #598 from varunbpatil/compiler_warnings

Remove compiler warnings
Merge pull request #611 from mstorsjo/test-multithreading

Explicitly request 2 threads when testing slice threading
Merge branch 'master' of https://github.com/cisco/openh264