Use pushd/popd instead of repeated `cd X/cd "${S}"` calls in comedilib-9999.ebuild.

Use pushd/popd instead of repeated `cd X/cd "${S}"` calls in comedilib-9999.ebuild.

This removes the assumption that the build process was in ${S} at the
beginning of the block.  It also fixes problems with glob expansion as
explained below:

From the Pathname Expansion section of bash(1), pathname expansion
happens after word splitting, and therefore before variable expansion.
A simple example of this is
  tmp $ echo "${PWD}/*"
  /tmp/*
when I had originally expected the same output as
  pushd /tmp/; echo *; popd
  /tmp /tmp
  file1 file2 file3 ...
  /tmp