makesrna was often generating source every build, but not updating the files because the contents wasn't changed.
this happened because makefiles would check makesrna.c and rna_*.c files were newer then rna_*_gen.c and force a re-generation.
Now ensure updating the files even if they dont change when makesrna.c or rna_*.c are newer then rna_*_gen.c files.
Another solution for this would be to run makesrna program for each C file for finer grained deps.
or remove file comparison checks but that would mean a change to any rna_*.c file would rebuild all.