Commit c0b774fbc65e3e406a38d37b02fffda7c5d3df26 originally added this
logic but it did not properly clear the C command line flags to empty
when a previous positional argument had command line flags, because it
never set the "previous" flag to true.

This fixes C compiler flags not being reset to empty when using the
build system and a second positional argument has no arguments after a
first positional argument has arguments.

Thanks to @squeek502 for finding this.