recipe: psycopg2 2.9.12 (+ flet-libpq 17.5)

[ndonkoHenri]

Adds the psycopg2 PostgreSQL driver for iOS and Android (closes flet-dev/flet#3204), its native dependency flet-libpq, and a small forge feature — requirements.host_build — that psycopg2 uses to avoid bundling a library it static-links.

Recipes

• flet-libpq 17.5 — PostgreSQL's libpq, built from the release tarball (client library only). Ships the static libpq.a + libpgcommon/libpgport archives, headers, and a relocatable pg_config shim, alongside a shared libpq.so. Cross-compile fixes: ac_cv_header_langinfo_h=no (Android API 24 hides nl_langinfo), strip quotes from CFLAGS/LDFLAGS (postgres bakes them into config_info.c string literals on iOS), and make all-lib + pre-touch libpq-refs-stamp (iOS _atexit false-positive).
• psycopg2 2.9.12 — a Cython C-extension that statically links libpq + OpenSSL into _psycopg.so (Pattern E, like pymssql). mobile.patch ships the *_shlib.a PIC archives, links OpenSSL + the extra archives unconditionally (psycopg2 only does so in finalize_darwin/finalize_linux, which the cross sys.platform matches neither), and reads pg_config from the PG_CONFIG env var.

Forge: requirements.host_build

A native lib a C-extension static-links is folded into the extension's own .so — needed at build time, but not at runtime. requirements.host, however, promotes every flet-* dep to the wheel's Requires-Dist, so the consuming app bundles the unused flet-lib wheel (~2.7 MB android / ~3.9 MB iOS of dead weight per app).

requirements.host_build installs the dep into the cross environment exactly like host (its opt/include/opt/lib wired into CFLAGS/LDFLAGS and pkg-config), but excludes it from Requires-Dist (the promotion loop only walks host). psycopg2 uses it for flet-libpq; pymssql → flet-libfreetds is a natural follow-on adopter.

This PR also rewrites the now-stale requirements.build/host schema descriptions, which carried chaquopy-heritage claims that the current builders don't implement (a cmake build keyword generating chaquopy.toolchain.cmake; openssl/sqlite/python host keyword special-cases; a $SRC_DIR/../requirements extraction path; a >= Requires-Dist pin). They now accurately describe the build-vs-cross environment, the install_root CFLAGS/LDFLAGS/pkg-config wiring, the {platlib}/{prefix} script_env placeholders, and the flet-*-only == Requires-Dist promotion.

Tested

• Builds green on Android arm64-v8a and iOS (arm64 device, arm64 sim, x86_64 sim),rrrrrrrr+ Python 3.12. (CI builds the full matrix.)
• Live on-device (Android arm64 emulator + iOS simulator): psycopg2 imports (libpq 170005, static-folded), then a real TCP + SCRAM-SHA-256 auth + SELECT round-trip agains PostgreSQL 17.10 succeeds on both platforms.
• host_build verified: the psycopg2 wheel METADATA no longer lists flet-libpq in Requires-Dist; the built app no longer bundles the flet-libpq wheel; and psycopg2 still connects + queries on-device with it absent — confirming the dependency was purely build-time.