build: Fix warning about space after trailing backslash
doc: Build with qt5 is possible again

This reverts commit 62568f9a1e98065bd51069649f23a337eab89f94.
Also add some documentation for `--with-gui`.
Merge pull request #3564

54d2b91 doc: Build with qt5 is possible again (Wladimir J. van der Laan)