Install ginac-excompiler in $LIBEXECDIR, not in $BINDIR.

Install ginac-excompiler in $LIBEXECDIR, not in $BINDIR.

...and make the GiNaC library aware of where it is installed.

The ginac-excompiler script is only invoked by GiNaC::compile_ex(...)
and serves no purpose on its own. In compliance with the FHS, it should
be installed in $LIBEXECDIR, not in $BINDIR. This also disburdens
distribution packagers from having to provide a manpage (which may be
required for all programs in $BINDIR).

The location for $LIBEXECDIR defaults to ${prefix}/libexec/. It may be
overwritten at configuration time. (Many distributions want to set it to
${prefix}/lib/ginac/.)