econf: Replace unnecessary 'case' statements with 'if's

Replace the 'case' statements used to match 'configure' output with
simpler pattern-matching 'if's.

Acked-by: Alexander Berntsen <bernalex@gentoo.org>
econf: Add EAPI-conditional arguments via array

Use a dedicated array variable to add EAPI-conditional arguments to
the configure script instead of prepending them to the command
parameters.
Fix off-by-one error in supported EAPI list

Fix the off-by-one error in construction of supported EAPI list that
resulted in EAPI 5 being considered unsupported.
Make eapi_is_supported() reuse _supported_eapis list

Make the eapi_is_supported() function use the generated list of
supported EAPIs rather than partial lists and integer comparison.
Apply 'nonfatal' to helpers only

Make 'nonfatal' modifier affect helpers only rather than disabling 'die'
completely.