`makesrna` was somewhat wasteful in causing rebuilds on MSBUILD for
files that have not changed.
This makes 2 changes to `makesrna.cc`
- the code in place to keep "make" out of trouble and regenerate
files regardless if they have changed or not has been disabled for non
"Unix Makefile" generators, as they do not appear to be needing this.
This brings the number of actions ninja takes down after a white-space
change in `rna_wm.cc` from 423 down to 107.
- `rna_prototypes_gen.h` was always written, this is somewhat wasteful
and has been changed to be only written when changed.
This brings the number of actions ninja takes down after a white-space
change in `rna_wm.cc` from 107 down to 11.
in total, this saves several minutes of build time for simple changes
to rna source files.
Ref !110664.
cc2093a25a