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.

mesonbuild/backend/ninjabackend.py

@@ -1920,8 +1920,16 @@ rule FORTRAN_DEP_HACK
         elem.write(outfile)
 
     def generate_ending(self, outfile):
-        targetlist = [self.get_target_filename(t) for t in self.build.get_targets().values()\
+        targetlist = []
-                      if not isinstance(t, build.RunTarget)]
+        for t in self.build.get_targets().values():
+            # RunTargets are meant to be invoked manually
+            if isinstance(t, build.RunTarget):
+                continue
+            # CustomTargets that aren't installed should only be built if they
+            # are used by something else or are meant to be always built
+            if isinstance(t, build.CustomTarget) and not (t.install or t.build_always):
+                continue
+            targetlist.append(self.get_target_filename(t))
 
         elem = NinjaBuildElement(self.all_outputs, 'all', 'phony', targetlist)
         elem.write(outfile)