https://gitlab.synchro.net/main/sbbs/-/commit/372fa69084a25dc894f391f9
Added Files:
src/doors/syncretro/COMPILING.md
Modified Files:
src/doors/syncretro/README.md xtrn/syncarcade/README.md
Log Message:
syncretro: document the build, per-platform, in COMPILING.md

SyncRetro was the only one of the five termgfx doors without a COMPILING.md. The README's Building section is three code blocks, which is enough to repeat
a build you have already done once and not enough to do the first one: it
names no prerequisites, no configure options, and does not say what the configure output is telling you.

The gap that prompted this is the *nix path specifically. Two of this door's artifacts are per-platform -- the binary and the libretro core -- and both are installed by scripts (deploy.js, getcore.js) that put them in an <os>-<arch> sub-directory rather than at the door root. A console first installed from a Windows host therefore looks complete while having nothing a *nix host can
run, and nothing said so. SyncArcade was in exactly that state.

So COMPILING.md leads with Linux/Unix: the prerequisites (including that termgfx needs a C++ compiler, and what libjxl and libsndfile each buy), the build, the two configure lines that report which tiers were found, the configure options, deploy.js and why the sub-directory exists, that the core
is downloaded rather than built and must be fetched once per platform, probe_core as the smoke test for a freshly installed core, and the tests. Windows keeps a short section pointing at build.bat and the README.

Also corrects xtrn/syncarcade/README.md, which told a sysop who had already installed a sibling console that they had the binary already. True only on
the platform they installed it from.

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