Basically just add a switch to `dm.exe` and the Linux equivalent that lets you inline compiler defines.

Assuming you're familiar with C/C++ compilers, a lot let you do `-DMYDEFINE=MYVAL` to add preprocessor defines to your compilation.

Currently, we (99% of SS13 servers) have a script in our repositories known as `dm.sh` which will do the same, but create a modified DME which prepends these at the top of the dme file, which isnt really a clean solution. Being able to natively do `-DCI_BUILDING` or `-DPRODUCTION` as part of build chains would be a godsend to reduce the need for external dependencies.