#5959 skyline: minor performance tweaks

[Han-Wen Nienhuys]

https://codereview.appspot.com/559950045

[Anonymous]

Fails make

...
Making lily/out/clef.o < cc
Making lily/out/beam-engraver.o < cc
/home/james/lilypond-git/lily/skyline.cc:159:47: error: no 'Real Building::shift_to_intersect(Real, Real) const' member function declared in class 'Building'
 Building::shift_to_intersect (Real x, Real y) const
                                               ^~~~~
Making lily/out/dynamic-engraver.o < cc
Making lily/out/rhythmic-music-iterator.o < cc
/home/james/lilypond-git/build/../stepmake/stepmake/c++-rules.make:4: recipe for target 'out/skyline.o' failed
make[1]: *** [out/skyline.o] Error 1
make[1]: *** Waiting for unfinished jobs....
/home/james/lilypond-git/build/../stepmake/stepmake/generic-targets.make:6: recipe for target 'all' failed
make: *** [all] Error 2