Fix CMake build
Dead include and comments.
assert(0) => llvm_unreachable
Update gitignore to be effective.

git ignore.
Factorize build settings in external xcconfig files.
Doing this greatly reduces the number of duplicated settings, as 'SDK' specific settings can be specified only once instead of once per Build Configuration.

This also fix a couple of issues with 10.9 SDK and python (which was already fixed for some targets but not all).
Project cleanup
Using static library for Xcode build.
Move LLDBWrapPython outside of source as CMake does not like it.
Potable Host MD5 implementation that does not rely on md5 command line tool.
Typo
auto_ptr -> unique_ptr
Dead code
Replace shell invocation by native function call.

mkdir

Whoops
Fix Windows pread/pwrite emulation, and write a comment about how it should be done to be thread safe.
Don't try to guess sys header condition, and simply check if each signal is defined.
Use thread safe variant.