Fixes to processing-make-compile-command

[KevOrr]

• Changed (symbolp 'cmd) to (symbolp cmd)
• Instead of shell-escaping bits of the full command and then gluing it together manually (which is error-prone), instead collect the argument list and then call combine-and-quote-strings
• Should fix the new discussion in https://github.com/ptrv/processing2-emacs/issues/6#issuecomment-356878540