ninja: Don't add every CustomTarget to 'all'
ninja: Don't add every CustomTarget to 'all'

Otherwise they are built regardless of whether they are actually used by
anything else. Only build them if they're going to be installed or
always-built.

Ideally, we should also do this with all BuildTargets, and provide
a mechanism for people to specify which targets they want built with
'all', and a way for people to add them to custom targets.. Without
this, things like tests and examples are *always* built with no way to
turn that off.

For now, we just do this because it also with tests that check for
dependency issues. Including all CustomTargets in `all` results in
dangling targets to also be built, which hides the problem and makes it
racy.

jpakkane/meson,fmuellner/meson,QuLogic/meson,ernestask/meson,mesonbuild/meson,becm/meson,pexip/meson,aaronp24/meson,jpakkane/meson,ernestask/meson,MathieuDuponchelle/meson,pexip/meson,MathieuDuponchelle/meson,fmuellner/meson,trhd/meson,ernestask/meson,pexip/meson,ernestask/meson,thiblahute/meson,becm/meson,ernestask/meson,aaronp24/meson,MathieuDuponchelle/meson,fmuellner/meson,mesonbuild/meson,rhd/meson,wberrier/meson,rhd/meson,QuLogic/meson,rhd/meson,jeandet/meson,thiblahute/meson,aaronp24/meson,wberrier/meson,rhd/meson,becm/meson,pexip/meson,centricular/meson,jeandet/meson,becm/meson,aaronp24/meson,trhd/meson,fmuellner/meson,trhd/meson,thiblahute/meson,becm/meson,mesonbuild/meson,aaronp24/meson,fmuellner/meson,QuLogic/meson,mesonbuild/meson,jeandet/meson,MathieuDuponchelle/meson,becm/meson,jpakkane/meson,aaronp24/meson,thiblahute/meson,thiblahute/meson,trhd/meson,trhd/meson,fmuellner/meson,trhd/meson,jpakkane/meson,rhd/meson,pexip/meson,jeandet/meson,thiblahute/meson,mesonbuild/meson,wberrier/meson,wberrier/meson,jpakkane/meson,QuLogic/meson,QuLogic/meson,trhd/meson,pexip/meson,aaronp24/meson,jeandet/meson,pexip/meson,MathieuDuponchelle/meson,fmuellner/meson,jpakkane/meson,MathieuDuponchelle/meson,becm/meson,trhd/meson,ernestask/meson,fmuellner/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,rhd/meson,jeandet/meson,centricular/meson,wberrier/meson,pexip/meson,ernestask/meson,becm/meson,QuLogic/meson,thiblahute/meson,rhd/meson,mesonbuild/meson,becm/meson,jpakkane/meson,QuLogic/meson,fmuellner/meson,centricular/meson,jeandet/meson,QuLogic/meson,jpakkane/meson,centricular/meson,jeandet/meson,centricular/meson,rhd/meson,trhd/meson,centricular/meson,thiblahute/meson,becm/meson,ernestask/meson,aaronp24/meson,pexip/meson,MathieuDuponchelle/meson,wberrier/meson,thiblahute/meson,aaronp24/meson,QuLogic/meson,centricular/meson,mesonbuild/meson,jeandet/meson,wberrier/meson,MathieuDuponchelle/meson,wberrier/meson,ernestask/meson,MathieuDuponchelle/meson,jpakkane/meson,mesonbuild/meson,centricular/meson