Add conditional for bootstrapping new architectures
    
    The libxcrypt package may change its build-system from autotools to
    mesonbuild sometime in the near future.  Thus we should be able to
    build a bootstrap package for ninja-build with the smallest possible
    build- and runtime-dependencies.
    
    With bootstrap enabled the only build-dependency, except for a
    working compiler-toolchain, will be re2c, and there will be no
    runtime-depencies needed to install the ninja-build package.
    
    Signed-off-by: Björn Esser <besser82@fedoraproject.org>