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