Sorry Kry, this is just broken.
"make install" currently rebuilds the sources because install depends on all, which depends on BUILT_SOURCES, which is always rebuilt. These 2 .h files trigger a rebuild of everything because mostly anything else depends on them. "make" followed by "make install" effectively builds the codebase twice. I just accidentally noticed it because the build took almost twice as long as usual.
I won't believe that this is intentional until you clearly say so. Otherwise, can we please try to fix it properly?