I don't think you've fixed anything using -f flag.
-f
--auxiliary name
When creating an ELF shared object, set the internal DT_AUXILIARY
field to the specified name. This tells the dynamic linker that
the symbol table of the shared object should be used as an auxil-
iary filter on the symbol table of the shared object name.
Go back to using -l and fix the path as it's obviously wrong.

Use absolute paths if you don't know how to use qmake variables.
Start a console, go to directory where you have the qwt library, use 'pwd' to get full path and use that path.