Should I setup 1 dev environment so I can compile everything one the same platform?
This might seem the most logical setup but maybe not:

• emscripten will always be a different compiler and a different path
• I use osX, Windows en Ubuntu, depending on what I'm doing ... I want to fiddle with code on each of those platforms.
  javascript is the same everywhere, but C compilers are not ... keeping a dev tool chain up and running on those 3 platforms might more work then its worth - use Docker maybe?
• I still have some real (physical) Amiga's ... it would be nice to code on them as well

Allthough there are some really interesting setup to compile amiga binaries on osX, widows and Linux, I ended up setting up a compiler on the amiga itself.

On the emulated amiga, I use a virtual harddisk that resides on Dropbox.
So whatever system I'm using, when I fire up an emulated amiga, it will always be the exact same setup on everyplatform because the hard disk file is synced.
Even more: after setup, I can just dump the content of the virtual harddisk onto a CF-card, pop it into a real amiga and BAM! everything is up and running there also (try doing THAT with a windows PC!)

All the source files are on Dropbox too, available both local and on the emulated amiga.
I can still use my editor of choice (Jetbrains FTW!) and compile it on the amiga.

If I'm REALLY eager, I can even run the virtual amiga on my Android phone - with the exact same virtual harddisk - and compile from there (OK, not very practical, but it's possible)

So let's keep Amiga compiling on the Amiga. 
We'll worry about mobile later.