Allow passing --support-longjmp flag to embuilder

[ryanking13]

Some ports (freetype, libpng, ...) output different lib names based on whether the SUPPORT_LONGJMP flag is set. It would be useful to set this flag in embuilder so users can build the exact version of the library that they want to use.

embuilder --support-longjmp=wasm build freetype

[sbc100]

The way to get variations in ports is to create "variants". Then you can name those variants on the embuilder command line.

For example you can do ./embuilder build freetype or ./embuilder build freetype-legacysjlj. See

emscripten/tools/ports/freetype.py

Line 11 in a56a99f

variants = {'freetype-legacysjlj': {'SUPPORT_LONGJMP': 'wasm', 'WASM_LEGACY_EXCEPTIONS': 1}}

[ryanking13]

Oh, I see. I wasn't aware that I can build variants like that. Thanks!