The sysroot determination is fairly complex and will soon also be needed when
building with meson. Instead of duplicating the logic, move it to a dedicated
shell script invoked both by configure and meson.
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/2180a97c-c026-1b6c-cec8-d6e499f97017@enterprisedb.com
Andres Freund