Prefix should follow the path scheme of the host machine

[bonzini]

The prefix follows the path scheme of the host machine rather than the build machine, since it is the path under which the program will be installed on the host.

However, configuring a cross build with --prefix=c:/Program Files/XXX currently fails in sanitize_prefix, and default_prefix() likewise uses the build platform instead of the host.

To Reproduce

$ cat meson.build 
project('sample', 'c')

$ cat cross 
[binaries]
c = 'x86_64-linux-gnu-gcc'

[host_machine]
system = 'windows'
cpu_family = 'x86'
cpu = 'x86_64'
endian = 'little'

$ meson setup  --cross-file=cross --prefix='c:/Program Files/XXX' build
meson.build:1:0: ERROR: prefix value 'c:/Program Files/XXX' must be an absolute path

Expected behavior
The prefix is accepted, and DESTDIR=foo meson install places files in foo/c:/Program Files/XXX.