Build: Remove the static/dynamic tricks.

Build: Remove the static/dynamic tricks.

Most distros want xz linked against shared liblzma, so
it doesn't help much to require --enable-dynamic for that.
Those who want to avoid PIC on x86-32 to get better
performance, can still do it e.g. by using --disable-shared
to compile xz and then another pass to compile shared liblzma.

Part of these static/dynamic tricks were needed for Windows
in the past. Nowadays we rely on GCC and binutils to do the
right thing with auto-import. If the Autotooled build system
needs to support some other toolchain on Windows in the future,
this may need some rethinking.