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

meson.build

@@ -16,6 +16,10 @@ project('postgresql',
   default_options: [
     'warning_level=1', #-Wall equivalent
     'buildtype=release',
+    # For compatibility with the autoconf build, set a default prefix. This
+    # works even on windows, where it's a drive-relative path (i.e. when on
+    # d:/sompath it'll install to d:/usr/local/pgsql)
+    'prefix=/usr/local/pgsql',
   ]
 )