Set cmake min. version to 2.8.10 (except on Mac).

This is a partial reversion of 8907c0f442fbff57711135a66220b0bcb870a506.
2.8.10 was chosen after build tests on Debian and Ubuntu.
Tentative fix for QVM under msvc 64bits
Fix duplicate cvar and commands caused by the game VM
Prefix the Cvars in common/cm
Use std::shared_ptr<Sample> in Sound and fix LoopingSound to actually loop
Fix env_afx_push: now players jump, delay after jump taken, entity toggles
Merge pull request #302 from neumond/master

Fix env_afx_push
Merge commit 'f6c03fae5ec0a3ef0422c924686c281db5debc87' into for-0.27
Audio resource handles: don't try to clear an invalid handle.
Add support for cmake versions less than 2.8.10
Add timings for VM loading
Generate stripped versions of nacl executables
Help text: briefly note changes to armouries and turrets.
Help text: adapt adv. dretch text for the mantis.
Help text: spelling corrections.
Help text: add info about no battery packs.
Include *-stripped.nexe in the base .pk3, renaming them appropriately.