Add input dependency on custom build step.

Patch by Stefan Krah.

Analysis by Victor Stinner. Patch by Stefan Krah.

to prevent constant regeneration of importlib.h.

Patch by Kristján Valur Jónsson.

Extract cross-platform condition variable support into a separate file and
provide user-mode non-recursive locks for Windows.

the dll importer on windows to fail.

If a dependency is expressed both in the .sln file and the .vcxproj file
msbuild is confused.  Removing the .sln dependencies which are
deprecated anyway.  See:
http://blogs.msdn.com/b/visualstudio/archive/2010/12/21/incorrect-solution-build-ordering-when-using-msbuild-exe.aspx

"missing dependency", causing pythocore to be rebuilt every time.

use "references" to link to dependent projects.
Update readme and batch files.