configure: differ between check and yes/no

configure: differ between check and yes/no

So far one could only specify yes/no (optionally for all drivers through
--enable-all-drivers={yes,no}). However, even when stating a =yes, the
dependency check would override it to a =no if the dependencies are
missing.

This is a bit annoying for scripts since a script may specify
--enable-foo-driver=yes but in fact the build may not contain
`foo-driver` due to missing build dependencies. This is especially
problematic for distro's which look like they currently do not build a
defined driver set - which results in random drivers missing [1].

This fixes that by following the autoconf examples [2] that allow a third
state `check`. With that `no` means no, `yes` means yes (raise an error
if deps are unavailable) and `check` gracefully disables drivers with
unavailable dependencies.

With that --enable-all-drivers can be used to ensure that all drivers are
built in. Certain drivers can still be explicitly disabled by overriding
the individual driver setting.

This way distros can specify --enable-all-drivers and new drivers with
unmet dependencies will cause an error rather than silently getting
dropped of the build.

[1] https://sigrok.org/bugzilla/show_bug.cgi?id=1699
[2] https://www.gnu.org/software/autoconf/manual/autoconf-2.67/html_node/External-Software.html#External-Software

To: sigrok-devel@lists.sourceforge.net
Cc: Daniel Thompson <daniel.thompson@linaro.org>