configure.ac: Avoid uname, which breaks cross-compilation

configure.ac: Avoid uname, which breaks cross-compilation

When cross-compiling on Linux, configure will misdetect the target as
Linux because it uses uname instead of relying on the $host variable.
This results in including libvirt_linux.syms into libvirt.syms and
therefore trying to export undefined symbols.

Replace uname checks with $host checks to fix this.