cgen: define L_tmpnam on glibc instead of including <stdio.h> (follow-up to #28121)

[medvednikov]

Follow-up to #28121, which fixed the glibc error: 'L_tmpnam' undeclared here build failure (#28108) by re-adding #include <stdio.h> on the glibc prelude path.

V tries to include as few system headers as possible, so this replaces that include with a direct macro definition.

Why just L_tmpnam

V declares the stdio functions manually and does not #include <stdio.h>. glibc only defines the stdio limit macros while <stdio.h> is being processed (they sit behind #ifdef _STDIO_H in <bits/stdio_lim.h>), and L_tmpnam is the only one of them that <stdio.h> itself uses in a declaration:

extern char *tmpnam (char[L_tmpnam]) __THROW __wur;

FILENAME_MAX, TMP_MAX, FOPEN_MAX are only #defines for user code — nothing in the tree or the generated C references them — so L_tmpnam is the one that must exist when a <stdio.h> gets pulled in later by a module header (sqlite3.h, gc.h, …).

#if defined(__GLIBC__) || defined(__GNU_LIBRARY__)
#ifndef L_tmpnam
#define L_tmpnam 20
#endif
#endif

20 is a stable glibc constant, and glibc's own definition is #define L_tmpnam 20 (identical), so if <stdio.h> is later processed its redefinition is a no-op — no -Werror redefinition diagnostic.

v3 (new compiler)

Verified the three #28121 issues are not a problem for the v3 backend:

• glibc L_tmpnam: v3 reuses this same c_headers block via manual_stdlib_c_headers() (embeds cheaders.v), so it inherits the define on its system-libc path; its headerless path never includes <stdio.h> at all (it forward-declares the functions it uses). Added a preamble_test.v assertion to lock this in.
• v doc -theme-dir / v missdoc -e: vdoc and missdoc are shared tools; v3 does not reimplement them.
• -e argument passthrough: v3 has no argument parser of its own — the launcher handling lives in pref.parse_args_for_launcher(), which every backend goes through.

Tests

• vlib/v/gen/c/cheaders_manual_stdlib_decls_test.v: asserts the glibc guard + L_tmpnam define are in the prelude.
• vlib/v3/gen/c/preamble_test.v: asserts the v3 backend inherits the define.

Both pass locally; the actual glibc compile is exercised by the Linux tools-linux job (it builds vbug-report.v, the original failing case).

🤖 Generated with Claude Code

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Master CI failures for cbe0fe208a1569091f9dc9b754f0cbde14ef878e

Found 4 failed workflow runs. Detailed diagnostic comments follow.

• CI macOS — run 32190095484
• CI Windows MSVC — run 32190095438
• CI Windows TCC — run 32190095359
• CI Windows GCC — run 32190095460

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Failed workflow: CI macOS

Run 32190095484; 1 failed job(s).

clang-macos (macos-14) — job 95882328072

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[github-actions]

Failed workflow: CI Windows MSVC

Run 32190095438; 1 failed job(s).

msvc-windows — job 95882327819

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[github-actions]

Failed workflow: CI Windows TCC

Run 32190095359; 1 failed job(s).

tcc-windows — job 95882327904

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[github-actions]

Failed workflow: CI Windows GCC

Run 32190095460; 1 failed job(s).

gcc-windows — job 95882327950

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