odbc: build the ODBC shim as a Windows DLL (Windows port, phase 2)

[lemenkov]

Phase 2 of the Windows port: re-enable src/odbc on Windows (Phase 1 #8 gated it off) and get it compiling and exporting as seerodbc.dll under MinGW-w64. Builds on #8 and the iconv fix #9.

The good news

The *W path needed no work — it already uses sizeof(SQLWCHAR) + UTF-16 throughout, which matches Windows' 2-byte SQLWCHAR. So the scary "UTF-16 vs UTF-32" landmine was a non-issue, and this is mostly headers, a few libc shims, and the PE export mechanism.

Changes

• src/odbc/odbc_platform.h (new) — pulls <windows.h> before the native ODBC headers (which use HWND/DWORD/GUID without defining them). Included ahead of <sql*.h> in odbc_internal.h, convert.h, unicode.c (convert.c gets it transitively).
• src/common/compat.{c,h} — three more portable libc replacements the Windows CRT lacks: seer_strndup (exec.c, sqlprep.c), seer_strcasestr (catalog.c), seer_getline (odbcini.c). Used on every platform (one code path).
• odbcini.c — seer_getline, and "r" instead of "re" (no O_CLOEXEC on Windows).
• src/common/log.h — check seer_log's format against the gnu_printf archetype on MinGW. The bare printf archetype there means msvcrt (no %z/%ll), so -Wformat wrongly rejected our %zu; we already build with __USE_MINGW_ANSI_STDIO, so gnu_printf is the matching, warning-clean choice. (Also silences the pre-existing tns %z warnings.)
• src/odbc/seerodbc.def (new) + meson — the PE equivalent of seerodbc.map/.exp. GNU ld applies version scripts only to ELF, so PE restricts exports via a .def. Verified the DLL exports exactly the 64 SQL* entry points and nothing else (no core/OpenSSL leakage). Names are undecorated so a future 32-bit __stdcall build still exports plain SQLConnect, not _SQLConnect@N.
• tests/meson.build — skip test_odbcini on Windows for now (the reader builds; only the test's mkdtemp/setenv harness isn't ported, and file-based odbc.ini matters less where DSNs live in the registry — both land with the later registry/DSN phase).

Verification

• Fedora mingw-w64 cross-toolchain (new local loop): full tree (core + shim) builds warning-clean; the DLL's export table matches seerodbc.def exactly (64 SQL*, nothing else).
• Native Linux: offline 9/0/4, live 11g integration OK under ASan/UBSan — the portable compat replacements are behavior-identical.
• The MinGW compile of the shim is validated by the Build + unit (Windows/MinGW) CI job here.

Next (Phase 3)

Loads under the MS Driver Manager + live query; InitOnceExecuteOnce; and the registry/DSN work (SQLGetPrivateProfileString) — which is also when test_odbcini gets its Windows port.

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