configure: parse --enable/--disable-strip automatically, flip default

Always include the STRIP variable in config-host.mak (it's only used
by the s390-ccw firmware build, and it adds a default if configure
omitted it), and use meson-buildoptions.sh to turn
--enable/--disable-strip into -Dstrip.

The default is now not to strip the binaries like for almost every other
package that has a configure script.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>