gnuplot: Improve startup performance

Before executing the gnuplot executable the environment variable `GDFONTPATH`
is populated with a list of font directories, which is obtained from `fc-list`.
In that process we iterated over each line and called `dirname` on it, which
introduces a performance hit for loading and executing the external executable
`dirname` every time.

The new version avoids the loop.

The author of this patch measured a 42 fold performance improvement:

old version:

    $ time ./gnuplot_old/bin/gnuplot -e ''
    real    0m3.828s
    user    0m0.392s
    sys     0m0.465s

new version:

    $ time ./gnuplot_new2/bin/gnuplot -e ''
    real    0m0.091s
    user    0m0.112s
    sys     0m0.014s

The correctness of the value of `GDFONTPATH` was confirmed with the following
command and comparing its output between versions:

    $ gnuplot -e 'print system("echo $GDFONTPATH")'

0 files changed, 0 insertions, 0 deletions