Pass -msse2 when using gcc/clang.
Update INSTALL regarding PYTHON variable.
Make failed tests output the exit code.
Add LIBDIR to LD_LIBRARY_PATH when running the Cython tests.
Try to locate python3 before python.
Revert "Update INSTALL regarding PYTHON variable."

This reverts commit a5f556e89711ec5703e7dee29ac86c0605321230.
Fix mistake in documentation
Merge branch 'master' of https://github.com/vapoursynth/vapoursynth.git
Add -start and -end arguments to vspipe