#461 allow -o to also specify a directory

[Simon Sobisch]

Currently we have -o path/to/filename, --save-temps (only storing intermediate data) and the special case -g which writes a bunch of files to the place of -o, which defaults to be the base name of the source file in the local directory.

The missing option is to have cobc prog.cob -g -o exe with "exe" being a directory.
In this case it is enough to check if exe is a directory, and if yes change that to be exe/prog (= SLASH_CHAR + basename of the input file).

Currently one receives an error from the C compiler/linker that this is a directory.