Re: meson: pass OpenSSL/ICU include dirs to extensions

Both follow-up patches are attached in order on top of your public v4.

Patch 1 handles CMake include directories returned as strings or lists
and escapes spaces in exported flags. Patch 2 keeps PostgreSQL headers
ahead of extra include paths, passes those paths to all six fallback
header checks, and exports them through PGXS and pg_config.

I applied v4 and these exact patches to PostgreSQL master
6b9f40d49ae33eb0b4a7c77666fe4971ef7ba087. On hosted Windows x64 and
ARM64, the include-export cases passed with Meson 0.57.2 and 1.11.2.
All six fallback header probes and the psql header-order check passed
on both architectures. The v4 control reproduced the wrong-header
failure.

Hosted run: https://github.com/Harrison-Booth/postgresql-windows-ci/actions/runs/39595041637

Patch 2 addresses the same header-order concern as CF6056, but also
covers fallback probes and exported flags. I haven't combined or
tested both implementations together. These focused checks don't cover
a full server, installed extension, or PGXS runtime.

Could you review the attached follow-up patches, especially patch 2's
header-order approach?

Best,

Harrison