fix(gnu.org/gcc): bottle the libc via --with-sysroot (addresses #8423)

[tannevaled]

Summary

• Adds gnu.org/glibc + kernel.org/linux-headers as Linux runtime deps
• Configures gcc with --with-sysroot={{deps.gnu.org/glibc.prefix}}, --with-build-sysroot=..., --with-native-system-header-dir=/include, --with-glibc-version=2.43
• Result: gcc's spec file embeds the bottled-glibc paths; end-user pkgx gcc test.c finds stdlib.h + crt*.o + libc.so.6 from the pkgx-integrated bottle, no host glibc-devel / libc6-dev needed (Fedora repro in Inconsistencies with dnf/apt install gcc #8423).

Why this approach (vs #13084's first attempt)

• fix(gnu.org/gcc): wire libc + linux-headers (closes #8423) #13084 added glibc as a dep + a runtime.env exporting CPATH/LIBRARY_PATH. That:
  • clobbered brewkit's composed CPATH during gcc's own build → gmp.h not found at configure
  • and (in a later iteration) caused fixincl.c to link-fail with undefined reference to __isoc23_strtoul (compile saw bottled glibc 2.43 headers, link saw host glibc ~2.35).
• This PR uses gcc's own --with-sysroot mechanism instead — same libc backs both compile AND link, no env-var override.

Pairs with

• fix(gnu.org/gcc): add Debian multi-arch triplet symlinks (addresses #8423) #13083 (multi-arch triplet symlinks) — together fully close Inconsistencies with dnf/apt install gcc #8423.

Test plan

• CI builds gcc on linux/x86-64 + linux/aarch64
• Verify pkgx gcc -print-sysroot returns the bottled glibc path
• Manual: on a stock Fedora without glibc-devel, pkgx gcc test.c -o test succeeds
• Darwin builds unchanged (sysroot args are linux-only)

🤖 Generated with Claude Code

[tannevaled]

Closing — both attempts (runtime.env CPATH, sysroot config) fail in distinct ways:

1. runtime.env CPATH (PR fix(gnu.org/gcc): wire libc + linux-headers (closes #8423) #13084): clobbered brewkit's composed CPATH during gcc's own build → gmp.h not found at configure.
2. --with-sysroot (this PR): configure-time test programs link against pkgx glibc but the embedded interp/ABI doesn't survive the brewkit-environment runtime, so configure rejects with cannot run C++ compiled programs. --with-build-sysroot made it worse (sysroot-stripped /lib interp path doesn't exist on the host); without it, brewkit's LD_LIBRARY_PATH pulls in pkgx glibc anyway and the libc/crt asymmetry trips configure tests.

The proper fix for #8423 needs a non-build-time mechanism. Two known paths:

• a thin pkgx gcc wrapper installed alongside the real binary, modeled on the bklibcvenv pattern from brewkit#348, that re-exports CPATH/LIBRARY_PATH for end-user invocations only (no build-time poisoning)
• a separate bin/sysroot-gcc shim that takes --sysroot={{pkgx.glibc.prefix}} and forwards to gcc, leaving the canonical gcc bottle untouched

Leaving the issue open and revisiting later. The 9 newly-merged green PRs (#13076 #13082 #13083 #13085 #13086 #13087 #13088 #13090 #13091) are independent of this.