Fixed extraction of $toreduce_base.

fileparse interpreted @suffixes as a list of patterns, and 'file.cc'
was split into 'file.' and 'cc' instead of 'file' and '.cc'
(because 'cc' matches '.c'), resulting in strange names of output
files (file..orig and file..best).

This fix also enables use of additional file extensions like '.h'
or '.ii'