Win32/NMake builds: Support builds from GIT (#498)

Win32/NMake builds: Support builds from GIT (#498)

Add Python scripts to generate the full win32/config.h.win32 and
src/hb-version.h which can be used to build directly from a GIT
checkout.  Since the scripts are currently intended for building from a
GIT checkout, these are not distributed in the release tarballs.

Also, support the re-build of Ragel-generated .hh headers using the NMake
build system, and allow one to specify the path of the Ragel executable
if a suitable one cannot be found in the PATH.

Update the Win32/NMake build documentation to let people know about how
these mechanisms can be utilized.