Limit spidermonkey building instructions to Linux only

[yurydelendik]

Per recommendations from https://bugzilla.mozilla.org/show_bug.cgi?id=1776255#c2:

I'd recommend switching to Linux to do it, and maybe working with the authors of the guide to get it updated to either explicitly support doing the build on hosts other than Linux, or explicitly stating that the guide is for Linux hosts only.

[ptomato]

Through ESR 91 the instructions work fine for me on MacOS, did something change?

[mgaudet]

Interestingly, I could not get ESR91 to build on my machine either. Complained about pkg-config being missing.

On trunk, eliding the --with-system-zlib flag did get it to build (which otherwise was complaining about pkg-config)

I got ESR 91 to build by brew installing pkg-config

[mgaudet]

I was also able to build the examples after that.

[ptomato]

zlib is already listed as a prerequisite, maybe we should add pkg-config to that list? https://github.com/mozilla-spidermonkey/spidermonkey-embedding-examples/blob/esr91/docs/Building%20SpiderMonkey.md#prerequisites