fix C compiler invocation for cygwin

The changes of commit c8003c2f lead to errors such as

;;; Internal error:
;;;   ** Could not spawn subprocess to run "gcc"..
2 [main] ecl 3176 child_info_fork::abort: address space needed by 'eclzshYn0.dll' (0x170000) is already occupied

due to cygwin having issues with forking.