disable automatic GDEF generation when building variable fonts

how can we turn off automatic GDEF generation when building variable fonts?

I’ve tried `--feature-writer None` and/or `--no-generate-GDEF ` – this works when generating a static font, but it does not work when generating a variable font.

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### Context

fontmake's automatic GDEF table generation assumes that all glyphs containing anchors with names starting with an underscore are marks (non-spacing combining glyph, GDEF class 3). such glyphs are not always marks though; there are other use cases for anchors, such as building glyphs from parts; and there is an older convention to use for example `spam` (in the base glyph) and `_spam` (in the component glyph) to attach component to base. Having `_spam` in a glyph like `/O` (to attach it to some other glyph) does not mean it should be treated as a mark.

- [simple test case](http://github.com/googlefonts/amstelvar-avar2/tree/main/DEBUG-GDEF)

Start with the DEBUG-GDEF simple test case and compare the --no-generate-GDEF and --feature-writer options between static and variable-font builds. Done means automatic GDEF generation can be disabled for variable fonts and the behavior is verified against that test case.

Written by the indexing model from the issue text.