Mac OS 10.9 SDK compatibility

- detect Mac OS 10.9 SDK
- set -DMACOS_10_9 flag and MACOS_10_9 variable
- use accelerate toolkit instead of vecKit
- langsource/GC: fix header path
Mac 10.9 SDK Compat: make >10.7 default and introduce switch

- don't try to detect 10.9 SDK as 10.10 will come some day
- vecLib is deprecated since 10.7 SDK
MacOS 10.9 SDK compat: remove all pre 10.7 compat
MacOS 10.9 SDK compat: lang/GC: remove unnecessary include.
Merge pull request #994 from mortuosplango/master

Mac OS 10.9 SDK compatibility

Seems to have been sorted, so merging