config: default option for enable-dri3 is not implemented

config: default option for enable-dri3 is not implemented

The first symptom is the help text:

  --enable-dri3     Build XCB DRI3 Extension (default: "$sendfds")

The implementation variable $sendfds leaked into the user interface.
Testing the various user inputs:
  <nothing>                DRI3 is enabled     PASS
  --enable-dri3		   DRI3 is enabled     PASS
  --enable-dri3=yes        DRI3 is enabled     PASS
  --enable-dri3=no         DRI3 is disabled    PASS
  --disable-dri3           DRI3 is disabled    PASS
  --enable-dri3=$sendfds   DRI3 is disabled    FAIL

This patch implements the usual idiom for features that are enabled by
default if the various conditions are met (sendfds is available).
New help text:

  --enable-dri3     Build XCB DRI3 Extension (default: auto)

With the additional user input:

  --enable-dri3=auto

which is equivalent to providing no input at all.

Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>
Signed-off-by: Uli Schlachter <psychon@znc.in>