Add helper batch file for testing on oculus rift.

Cleanup: Fix build error with MSVC

space after is not allowed

Cleanup: Fix build warnings with MSVC

gflags emits a few unused variable warnings since the main
CMakeLists.txt raised the warning from w4 down to w3. This
restores it back to w4 in the remove_strict_flags macro.

Cleanup: Fix build warnings with MSVC.

ffmpeg defines some of the math constants if they are not
found before including any of its headers, this lead to
a build warnings about M_E, M_LN2 and M_SQRT1_2 being
redefined once BLI_math_base.h gets included.

MSVC: Don't share pch between debug and release builds.

Precompiled headers were sharing the PCH file between debug and
release builds which is 'bad'. Adding the configuration to the
path fixes the issue.

Reported on chat by @mano-wii

Windows: Clear PYTHONPATH variable in various debugging batch files.

The PYTHONPATH environment variable sometimes causes issues,
clearing it in the helper batch files is easier than talking
every user individually though the process of removing it
to see if it resolves the issue they are having.

Fix: Build error with clang on windows.

clang does not seem support the static_assert with
message overload.

cmake/msvc: Add ninja support for precompiled headers

Ninja was unable to see the dependency between the cpp
that generated the pch and the compile units that used
it. Explicitly managing this now makes precompiled headers
work with both msvc and clang, with both msbuild and ninja
based generators.

Fix: Build error with ninja on windows

Ninja has issues detecting the implicit dependency on the
precompiled header output for freestyle. Disabled ninja
support for now until a proper solution can be found.

Cmake/Windows: Fix numpy being unpacked every build.

Every time you build cmake it unpacks numpy. It is seemingly
very unhappy with OUTPUTS being directories rather than files.

The creating of the directory is not needed, so that step was easy
to resolve, next problem: was there was not a target that outputs
the tarball so it too was deemed out of date. Losing that dependency
as well, fixes the issue, while this is not great if we ever update
the libs in svn, we are planning to drastically change the way we
package python for windows so this is ok for now, but needs to be
addressed with the next python update, T65547 is the tracking ticket
for this.

Tested with VS2015/2017/2019 with both msbuild and ninja generators