build: hoist system-specific checks before library checks

build: hoist system-specific checks before library checks

Commit f92c7e3 fixed a regression for native builds, but introduced
a regression for cross-compilation builds; in particular,
./autobuild.sh on a Fedora system with mingw cross-compiler fails
with:

checking for qemu-kvm... /usr/bin/qemu-kvm
checking for yajl_parse_complete in -lyajl... no
checking for yajl_tree_parse in -lyajl... no
configure: error: You must install the libyajl library & headers to compile libvirt

Since we default $with_qemu to 'yes' rather than 'check', and then
flip that default based on platform-specific checks, those platform
specifics need to come prior to any library checks that depend on
the value of $with_qemu.

* configure.ac: Ensure system defaults are sane before checking
for things that make decisions based on system default.

Signed-off-by: Eric Blake <eblake@redhat.com>