Fix build on some systems : don't pass libraries to mkproto.awk

Fix build on some systems : don't pass libraries to mkproto.awk

Currently, some static libraries have gone into object lists
that are also passed to make proto. (This should probably be
changed...) Then some awk versions fail when passed an non-
existing lib/libtdb.a e.g.. This patch changes mkproto.sh to
exclude \.a files from the list of files to process.

Michael
(This used to be commit 826b9ab07b58ca39350cca921002a4213ce7c7c9)