Use g_spawn_sync () instead of system (). Fixes #1429.

Use g_spawn_sync () instead of system ().  Fixes #1429.

Avoid the opening of a DOS box, console window, command window or
power shell during PostScript to PDF conversion on some versions of
Microsoft Windows [with GUB 6976be5 or newer].

Bypass the shell, thus avoiding quoting (think spaces or other special
characters in a file name), mangling and shell-incompatibilities.