The goal here is to pass an installation prefix only to the install step. It's not good enough to run cmake like this:
cmake -DCMAKE_INSTALL_PREFIX=$prefix ...
That would reconfigure the project with a prefix, possibly influencing
where the project thinks it will be installed at runtime, ie like
what ./configure --prefix=$prefix does in an autoconf based project.
However, If you look at the ninja rule for install, all it does
is call a cmake script. So instead of saying ninja install, do this:
cmake -DCMAKE_INSTALL_PREFIX=$DSTROOT/$prefix -P cmake_install.cmake
Note that this is different than DSTROOT in that it entirely replaces the
old prefix, so you need to include both the DSTROOT and the runtime prefix
in CMKAE_INSTALL_PREFIX when you do this.