The no git code also check for an existing version.h
The no git code also check for an existing version.h

My previous patch always used an empty file if git was not
available. This patch extends this to use the existing
version.h file if it exists (snapshot, etc.)

themperek/iverilog,CastMi/iverilog,themperek/iverilog,themperek/iverilog,CastMi/iverilog,CastMi/iverilog