More reliably handle make install from read-only directory
More reliably handle make install from read-only directory

commonly, the make install is run in a compiled directory by a
different user (i.e. an administrator) that doesn't necessarily
have write access to the build directory. This patch fixes it so
that the test that the version.h is up to date doesn't need to
write a temporary file in the build directory.

It also makes sure that if there is a real need to update the
verion.h, and it is not writable, then properly fail the build.
themperek/iverilog,CastMi/iverilog,CastMi/iverilog,themperek/iverilog,CastMi/iverilog,themperek/iverilog