Move PD_BUILD_VERSION from s_stuff.h.in to s_version.h.in

#397

Relevent makefiles and source files that use the PD_BUILD_VERSION variable are also updated. Not sure if I got all the comments updated. Tested with a clean compilation on Windows 10 and the build succeeded smoothly.

I also noticed that, in the repo, there are a few files (that seem to be) generated during build phase and should be ignored, including:

• externals/OSCx/libOSC/Makefile
• externals/OSCx/send+dump/Makefile
• externals/OSCx/src/Makefile
• externals/build/include/g_all_guis.h

Is my understanding correct? Should I remove them and add them to .gitignore?