build.bat: Fix /GetVersion after recent changes
UpdateChecker: work around git-describe issue
Update checker: Hide the revision number if it's 0 for consistency.
LAV Filters: Use out-of-source-tree builds for FFmpeg.

This way x86 and x64 compiled objects can coexist peacefully.
Internal LAVVideo: HW Acceleration wasn't correctly turned off on XP when none was detected.
Update Unrar to unrarsrc-5.0.12.tar.gz.
Update Changelog.txt.
Mention Windows 8.1 in the docs.
Update Readme.txt.
Switch back to GCC 4.7.3 due to issues with EMET.
build.bat: allow using /analyze with x64
Disable the C4005 warnings with /analyze.
Put GCCInfo compiled object into the arch-based folders for consistency.
Internal LAV Splitter: The advanced subtitle selection modes were ignored.

This commit fixes #3530.
Convert relative paths to absolute in commandline

Later we send the whole commandline through WM_COPYDATA,
and since current directories may differ, issues arise.

Fixes #3546