Fix #42

Context

See: https://github.com/gnuplot/gnuplot/blob/branch-5-4-stable/configure.ac#L121-L162
As you can see, the steps are as follows:

1. ./configure sets with-latex=yes as a default value
2. Check if with-latex=yes is really true
3. If neither TEXDIR nor path to the kpsewhich (kpsewhich is the command which returns a TEXDIR) are given, this install process fails.

If you pass the step 3 without setting TEXTDIR manually, permission issue may happens because kpsewhich sometimes returns a dir that is owned by root.

Changes proposed in this PR

• CustomBuilder.pm6 explicitly sets TEXDIR by --with-latex and --with-texdir=... arguments; and
• Make sure TEXDIR isn't owned by root (by using the subdir undir $prefix)