Fix leftover of convertpath removal

Tools/convertpath was removed in favour of cygpath in f8d198bf6, but configure
still called it when computing SWIG_LIB_WIN_UNIX on MinGW:

  ./configure: line 16298: ./Tools/convertpath: No such file or directory

SWIG_LIB_WIN_UNIX was left empty as a result. It is the only search directory
pointing at the autotools install location in a native Windows build, as SWIG_LIB
is derived from the executable location there, so a MinGW build installed to a
Unix style prefix could not find the SWIG library files.

Quote SWIG_LIB when passing it to cygpath, matching the other $CYGPATH -m uses,
so that a prefix containing a space is converted as a single path.

Closes #3534

Assisted-by: Claude Code (Opus 5)