darwin: Fix install name of libOSMesa

darwin: Fix install name of libOSMesa

Passing -module to glibtool causes the resulting library to be called
libSomething.so rather than libSomething.dylib on darwin.

Regardless if libOSMesa is a library or a module, it has been used as
the former for quite some time. Update the build to reflect that and
resolve the naming issue.

Cc: "10.5 10.6" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
[Emil Velikov: Tweak the commit message.]
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>