meson: Add prefix=/usr/local/pgsql to default_options

meson: Add prefix=/usr/local/pgsql to default_options

autoconf set PREFIX to /usr/local/pgsql, so using the same default for meson
makes sense. The effect on windows is that installation defaults to installing
to C:/usr/local/pgsql rather than meson's default of C:/, which doesn't seem
perfect, but OK enough.

Signed-off-by: Junwang Zhao <zhjwpku@gmail.com>
Author: Junwang Zhao <zhjwpku@gmail.com>
Discussion: https://postgr.es/CAEG8a3LGWE-gG6vuddmH91RORhi8gWs0mMB-hcTmP3_NVgM7dg@mail.gmail.com