Partially revert "automake: allow only shared builds"

Partially revert "automake: allow only shared builds"

Evidently at least static OSMesa is still used as shared one
causes substantial increase in the load time for some programs
that use it (from seconds up-to ~30min).

Rather than forcing everyone to use shared mesa, revert commit
a6efbac9fb502c4f0166e7a0680b6828e1f6926c and default to shared
build when both shared and static are disabled.

v2: Whitespace cleanup, drop silly comment.

Reported-by: Burlen Loring <burlen.loring@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>