Running `thunderbird -g` produces the following error:

    INFO  -> Starting Thunderbird with GDB ...
    INFO  -> LANG= /usr/bin/gdb -ex "handle SIG38 nostop" -ex "handle SIGPIPE nostop" -ex "run" /usr/lib/thunderbird/thunderbird
    /usr/bin/thunderbird: line 254: /usr/bin/gdb -ex "handle SIG38 nostop" -ex "handle SIGPIPE nostop" -ex "run" /usr/lib/thunderbird/thunderbird : No such file or directory

This occurs because exec is passed a single argument, which it
interprets as a file name that does not exist.  Instead, use the
following steps:

1. Build a command string which combines ${TB_GDB_DEFAULT_OPTS} and
   ${TB_ARGS[@]} (using `printf ' %q'`).
2. Print the command string with output_info.
3. Run it using `eval` (for word splitting) and `exec` (to replace the
   shell process with gdb, as before).

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>