1. Since the Ada compiler cannot be bootstrapped from most system
compilers, it must be at least as easy, if not easier, to
cross-compile it from a system which does have GNAT installed. At
a minimum, it must be possible to easily build a cross Ada compiler
for the other system, even when the other target is a host
(including hosts for which binutils is not readily available, like
AIX and Darwin), and then use that to build a native Ada compiler
for the other system.
2. Since the Ada compiler and tools must be installed in binary form
on the other system (because there is no compiler to build it from
source), they must be fully relocatable, that is they must be able to
be installed in any location; because the person doing the
installation may not be able (or willing) to write it to /usr/local
or wherever it was originally built for. (I know the C compilers
already do this, I am not sure if it really works for Ada).