configure: speed up check_deps()

x4 - x25 faster.

check_deps() recursively enables/disables components, and its loop is
iterated nearly 6000 times. It's particularly slow in bash - currently
consuming more than 50% of configure runtime, and about 20% with other
shells.

This commit applies few local optimizations, most effective first:
- Use $1 $2 ... instead of pushvar/popvar, and same at enable_deep*
- Abort early in one notable case - empty deps, to avoid costly no-op.
- Smaller changes which do add up:
  - Handle ${cfg}_checking locally instead of via enable[d]/disable
  - ${cfg}_checking: test done before inprogress - x2 faster in 50%+
  - one eval instead of several at the empty-deps early abort path.

- The "actual work" part is unmodified - just its surroundings.

Biggest speedups (relative and absolute) are observed with bash.

Tested-by: Michael Niedermayer <[email protected]>
Tested-by: Helmut K. C. Tessarek <[email protected]>
Tested-by: Dave Yeo <[email protected]>
Tested-by: Reino Wijnsma <[email protected]>
Signed-off-by: James Almer <[email protected]>

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