Build claims to be done when it isn't, unnecessary rebuilds of Core after touching bootstrap source files

1. Clone qtbase, configure and build it:

   git clone --depth 1 git://code.qt.io/qt/qtbase.git
   

   cd qtbase
   

   ./configure -platform linux-clang -opensource -confirm-license -nomake examples -nomake tests -prefix $PWD
   

   cmake --build . --parallel && echo success
   

   This will finish and write:

   success
   

2. Be paranoid. Make sure the build really finished:

   cmake --build . --parallel && echo success
   

   You will see:

   ninja: no work to do.
   success
   

   Nice.

3. Update the timestamp of a file:

   touch src/corelib/io/qurl.h
   

4. Rebuild:

   cmake --build . --parallel && echo success
   

   This will build most files again. Sooner or later it will finish and write:

   success
   

5. Be paranoid. Make sure the build really finished:

   cmake --build . --parallel && echo success
   

   It will start building files again. 53 steps for me.
   Huh?

Either the build didn't finish properly in step 4 or it's doing pointless rebuilds in step 5. Either step 4 should report a build failure or step 5 shouldn't trigger any rebuilds.