Changed the 'newest_LLVM' variable to 3.5 and fixed an error in help message
Merge pull request #872 from ncos/alloy_modifications

Changed the 'newest_LLVM' variable to 3.5 and fixed an error in help message
Merge pull request #871 from ncos/knc-backend-merge

Replaced vec16_i1 type definition in knc.h from typedef to struct version, adding a constructor
KNC prefetch:
- make L3 prefetch to hit L2$, instead of being nop.
- fix vector prefetch to use single intrinsic, instead of two (this is
  caused by bug in Composer 14.0 documentation).
Merge pull request #873 from dbabokin/knc

KNC prefetch:
Adding --with-gcc-toolchain alloy switch, which is passed to configure.
This is the only way to build functional clang on old Linux system,
which has old system gcc and new gcc (4.7+) installed in non-system
location.

Also adding switch to build only x86 target in clang.
Adding default sysroot detection for MacOS 10.9 (and newer), which
allows working out of the box. Otherwise C standard headers are not
found.
Merge pull request #874 from dbabokin/alloy-gnu-toolchain

alloy.py script upgrade