Fixes a struct stat size mismatch on 64 bits machines that made Node crash with a EXC_BAD_ACCESS on startup. Fixes #2061 for gyp builds. Solution proposed by Paddy Byers.
The "conditions" block is already within a "target_defaults", so its children amend target default settings already.
Hoist common settings into common.gypi. Restrict v8's common.gypi to v8 projects. Ensure v8 doesn't use /MP in debug builds. Add basic settings for other platforms. Make uv import common.gypi properly. Remove LTCG warning.