compile_et: Don't overflow input file buffer

compile_et: Don't overflow input file buffer

Don't overlow the buffer that's used for the input filename by
copying in to much with sprintf. Use asprintf to dynamically
allocate a buffer instead.

Link roken for rk_asprintf where needed.

Build compile_et with libtool, to ensure that it is linked statically,
as is needed for build tools such as compile_et.  (This requires
a preceding change to set a buildtool_roken make variable.)

Caught by coverity (#985907)

Change-Id: I207dd2c49bcae3f04fa41c826b08a0a615d5f422
Reviewed-on: http://gerrit.openafs.org/9545
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>