Building the compiler in 2 seconds with `dmd -i`

[Dennis]

In theory, dmd is really easy to build: it's just a collection of 
D files in a single repository with no external dependencies. Why 
have there always been such complex makefiles/scripts to build it?

Well, after replacing most of dmd's backend's C-style extern 
function declarations with proper imports [1], there's a simple 
way to build the compiler. Instead of needing to list every 
source file, you can leverage the `-i` flag which automatically 
compiles imports, and create a build script as short as this:

```
echo -n "/etc" >> SYSCONFDIR.imp
cd compiler/src
dmd -i dmd/mars.d -of=../../dmd dmd/eh.d dmd/backend/dtype.d 
-version=MARS -Jdmd/res -J../..
```

On my linux pc, these 3 lines take 2.0 seconds to complete and it 
uses max 680MB RAM.

The official way to build a non-release compiler is:

```
cd compiler/src
rdmd build.d BUILD=debug
```

build.d is 2400 lines, takes 2.3 seconds to complete a clean 
build, and uses max 613MB RAM.

The build script does separate compilation, multi-threading, 
includes debug info (`-g`), and builds other things as well. 
Still, my takeaways are:

- The build script is overly complex
- `dmd -i` is awesome
- Without too much templates/CTFE, dmd compiles pretty fast (150 
KLOC/second in this case), reducing the need for 
incremental/parallel compilation

Eventually I hope to make just `dmd -i dmd/mars.d` work, I'll see 
how close I can get to that.

[1] 
https://github.com/dlang/dmd/pulls?q=is%3Aclosed+is%3Apr+author%3Adkorpel+label%3ABackend+label%3ARefactoring+prototypes