C: auto-search pxx's crtl headers by default (+ -nostdinc)

• Type: feature (C frontend / include resolution) — Track A/C
• Status: done
• Owner: —
• Opened / Closed: 2026-06-27 (M5 sqlite bring-up, [[feature-c-desktop-lua-sqlite-path]]).

Problem

pxx searched only user -I roots for <> C includes, then fell back to /usr/include. So #include <stdarg.h> resolved to the host header (wrong ABI, doesn't map to pxx's __builtin_va_*) unless the caller manually passed -Ilib/crtl/include. That bit the sqlite bring-up (a false "invalid symbol in lea") and is a reproducibility footgun — real C programs need pxx's own freestanding/hosted headers on the default path, like any C compiler.

No new language builtin was needed — __builtin_va_arg/va_start/va_end already exist. The gap was purely the default include search path.

What changed

• AddDefaultCIncludeDirs (cpreproc.inc): for .c inputs, auto-registers lib/crtl/include as a default <> root — both ExeDir-anchored (<exedir>/../lib/crtl/include, robust to CWD; ExeDir = <root>/compiler/) and CWD-relative (lib/crtl/include/), mirroring the Pascal PAL-dir auto-add. Appended after user -I, so -I still wins (gcc semantics).
• -nostdinc / --nostdinc (NoStdInc): suppresses both the crtl default and the /usr/include host fallback — for freestanding/ESP/bare or pure builds (the canonical gcc flag name).
• The /usr/include fallback (cpreproc.inc) is now also gated by not NoStdInc.

Result

• test/cvarargs_int_b49.c and sqlite compile without the manual -Ilib/crtl/include. sqlite now reaches its next real wall (xAltLocaltime function-pointer struct field) on a bare invocation.
• -nostdinc correctly makes <stdarg.h> unresolved.
• The Makefile's explicit -Ilib/crtl/include on C tests is now redundant (harmless; left in place).

Future (noted, not built)

Tier the headers: freestanding (stdarg/stddef/stdint/stdbool/limits/float) always on; hosted (stdio/stdlib/string) only in hosted mode — matters for ESP/bare. One dir auto-added is enough for the desktop sqlite/lua path now.

Log

• 2026-06-27 - Implemented default crtl <> search + -nostdinc; gated (self-host byte-identical) and committed. User: "no external libraries needed" — pxx ships its own headers; the /usr fallback stays only as a last resort and is off under -nostdinc.