configure: use LIB_EXT rather than hardcoded .so

configure: use LIB_EXT rather than hardcoded .so

Some platforms different library extension - dll, dylib, a.
Honor that when we are creating the required links.

Rename LIB_EXTENSION to LIB_EXT while we're here.

With libglapi linking aside, building classic drivers on
non-linux platforms should be possible now.

v2: Resolve conflicts.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>