Build and test against CUPS 2.4, 2.5 and 3.x across 4 architectures

[rkt0209]

This brings libppd up to the multi-CUPS testing standard: it now builds and passes its test suite against CUPS 2.4 (distro libcups2), CUPS 2.5 (OpenPrinting/cups master) and 3.x (OpenPrinting/libcups master), on amd64, arm64, armhf and riscv64 — a 12-combination CI matrix.

Since libppd needs libcupsfilters too, the source-CUPS legs build libcupsfilters (and pdfio) from master against the active CUPS.

CI / build system

• .github/workflows/build.yml: 4 arch × 3 CUPS matrix (amd64/arm64 native, armhf/riscv64 under QEMU); runs make check + the downstream DESTDIR autopkgtest for every combination.
• ci/ci-setup.sh: per-combination setup — provisions each CUPS, pdfio and libcupsfilters from source where needed, then configures and tests libppd.
• configure.ac: detect CUPS 2.5 via pkg-config cups (2.5 dropped cups-config), keeping the existing cups3 and cups-config paths.
• libppd.pc.in: export @CUPS_CFLAGS@ so downstream consumers find CUPS headers when CUPS is installed from source.

Source-portability fixes

• C++ utilities (ppdc, ppdhtml, ppdi, ppdmerge, ppdpo, genstrings) used *_CFLAGS, which Automake ignores for C++ sources, dropping the CUPS include path on source-installed CUPS — switched to *_CXXFLAGS.
• libcups3 now defines its own cups_bool_t enum, colliding with libppd's helper alias — renamed the alias to ppd_bool_t.
• cupsParseOptions() is 4-arg on libcups3; added the translation in libcups2-private.h (2.5 → cupsParseOptions2, 2.4 → 3-arg form).

Test-data / environment fixes

• ppdFilterUniversal now defaults the text→PDF charset dir from CUPS_DATADIR (libcupsfilters master no longer falls back to it), and the test harness seeds the charset files into the staged data dir.
• All CUPS directory macros (CUPS_DATADIR, fontpath) now check the matching env var before the compiled-in path, for sandboxed/relocated installs.
• testppd's run-time LANG-switching localization cases are guarded for libcups2 only: libcups3's cupsLangDefault() caches the language, so mid-process locale switching no longer applies (test_ppd_localize still covers localization on libcups3).

Notes for upstream review

• libcupsfilters master texttopdf dropped its CUPS_DATADIR fallback — affects any caller that doesn't set the data dir.
• libcups3 cupsLangDefault() caches the language; run-time locale switching is no longer honored.