LDC: Speed up incremental builds with object file caching

[Nordlöw via Digitalmars-d-announce]

On Saturday, 17 September 2016 at 18:46:26 UTC, Johan Engelen 
wrote:
> I just finished another post about LDC:
> https://johanengelen.github.io/ldc/2016/09/17/LDC-object-file-caching.html

Great innovative work!

One thing: I'm missing an option for only doing time-stamp 
checking of the dependencies similar to how SCons checks if 
cached files are obselete.

See the Decider function documented here:

http://www.scons.org/doc/HTML/scons-user/ch06.html#idp1416061700

It can be either

- Decider('MD5')
- Decider('timestamp-newer')
- Decider('timestamp-match')
- Decider('MD5-timestamp')

As I see it the LDC-enhancement always does content hashing, that 
is, it is similar to SCons' `Decider('MD5')`. If so, it would be 
very worthwhile checking how much faster an approach similar to 
`Decider('MD5-timestamp')` would be. This approach only hashes 
content of dependencies if they have been modified since last 
hashing during last build of that object with the same compiler 
flags was done.