Sanitize WRKSRC by replacing mulitple occurences of '//' by '/'.

[tcberner]

Summary

If WRKDIRPREFIX contains a trailing slash there are issues, as seen in
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=199297

I assume this fix is a little bit too simple...

devel/qt5-qmake  %  make -VWRKSRC WRKDIRPREFIX=/tmp
/tmp/usr/ports/devel/qt5-qmake/work/qtbase-opensource-src-5.7.1
devel/qt5-qmake  %  make -VWRKSRC WRKDIRPREFIX=/tmp/
/tmp/usr/ports/devel/qt5-qmake/work/qtbase-opensource-src-5.7.1
devel/qt5-qmake  %  make -VWRKSRC WRKDIRPREFIX=/tmp//
/tmp/usr/ports/devel/qt5-qmake/work/qtbase-opensource-src-5.7.1

[mat]

I would rather the WRKDIRPREFIX documentation at the top of the file be changed to say MUST NOT INCLUDE A TRAINLING SLASH

[tcberner]

Does the reinplacement of // -> / impose too much of a computational hazard for it to be practical?

[tcberner]

Add comment in the documentation, rather than sedding it.

[mat]

I like that one better :-)