added debugging via gdb on Mac

[tfchurch]

There are intended to be two differences between the execution of gpp-compiler:compile and gpp-compiler:gdb:

1. the latter case compiles with "-g";
2. when gpp-compiler.runAfterCompile is set, in the former case the program itself is run, whereas in the latter case gdb is run on the program.

However on Macs, the code to run gdb was absent, so the 2nd did not occur: when gpp-compiler.runAfterCompile is set, the program itself is always run and gdb never is.

This branch fixes this. Annoyingly, it is not possible to call open on gdb directly because non-apps cannot receive arguments via open. The best workaround seems to be to create a temporary script which can be run via open.