Build system: Use common DLL install area

We have an issue with the PATH becoming too long, with a DLL build we need to have appropriate areas added in DllPath.bat and even with better dependency management it may still get a bit long for some IOCs. EPICS has the option to specify an alternative installation directory rather than $(TOP) which would put all db, dll etc. into one area. I’m not sure about putting all dbs together, I prefer them kept separate as it is more modular and avoids a name clash. With regard to DLLs a name clash is a problem whether they are in one directory or separate. So what I was proposing was:

* Modify EPICS build rules to additionally install all DLLs to a single area which can then be referenced by runIOC.bat
* Regularly run a repository check script for potential DLL name clashes in jenkins

The procedure outlines in #2320 doesn't completely solve this, it reduces the path via reducing dependencies but you can still overflow the path if you have a lot of dependencies. Epics v4 and area detector add quite a few things for example. However #2320 allows faster and potentially parallel builds, so is still useful to do.