configure: Override the os default with --disable-pie

configure: Override the os default with --disable-pie

Some distributions, e.g. Ubuntu 19.10, enable PIE by default.
If for some reason one wishes to build a non-pie binary, we
must provide additional options to override.

At the same time, reorg the code to an elif chain.

Reviewed-by: Alex Bennée <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
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v3: Update for QEMU_LDFLAGS.