fix(build): scope #235 compile-edge depfile to GCC; e2e 118 requires gcc

The awk filter strips GCC -fmodules' reversed make-rules; Clang's module
(.pcm) depfile shape differs and applying the GCC-shaped filter there is
untested. macOS CI (Clang) failed e2e 118 because the purview-rebuild depfile
didn't behave. Gate posixDepfile to CompilerId::GCC — Clang/MSVC keep the
pre-#235 behavior (no compile-edge depfile; header/purview rebuild tracking on
Clang is a follow-up, same as 0.0.96 → not a regression). e2e 118 now declares
`# requires: gcc` so it skips where GCC isn't the toolchain (macOS has no gcc
capability per run_all.sh). Linux GCC: depfile active, 117/118 green.

Author: Sunrisepeak

src/build/ninja_backend.cppm

@@ -456,7 +456,19 @@ std::string emit_ninja_string(const BuildPlan& plan) {
     // below), so a non-MSVC Windows toolchain keeps the pre-#235
     // behavior (no depfile) rather than depend on an unavailable filter —
     // msvcDeps (cl.exe) is unaffected either way (deps=msvc, no -MMD).
-    const bool posixDepfile = !msvcDeps && !mcpp::platform::is_windows;
+    //
+    // GCC-only: the awk filter strips the "reversed" make-rules that GCC's
+    // `-fmodules -MMD` bolts onto a module-TU depfile (`gcm.cache/<m>.gcm: |
+    // <obj>` etc., which ninja rejects as "inputs may not also have inputs").
+    // Clang's module system (.pcm) does not emit those and its module-TU
+    // depfile shape is different; applying the GCC-shaped filter there is
+    // untested and could yield a wrong dep set. Until Clang is verified,
+    // scope this to GCC — Clang keeps the pre-#235 behavior (no compile-edge
+    // depfile; header/purview rebuild tracking on Clang is a follow-up, same
+    // as it was on 0.0.96, so this is not a regression). e2e 118 declares
+    // `# requires: gcc` and skips where GCC is not the toolchain.
+    const bool posixDepfile = !msvcDeps && !mcpp::platform::is_windows
+        && plan.toolchain.compiler == mcpp::toolchain::CompilerId::GCC;
     const std::string mmd_flag = posixDepfile ? "-MMD -MF $out.d.raw " : "";
     const std::string mmd_filter = posixDepfile
         ? " && awk 'NR==1{print;next} /^[^ ]/{exit} {print}' "

tests/e2e/118_purview_include_rebuild.sh

@@ -1,6 +1,8 @@
 #!/usr/bin/env bash
-# requires:
+# requires: gcc
 # mcpp#235: compile edges must track header/purview/GMF `#include`s via a
+# GCC-only depfile (the awk filter strips GCC -fmodules' reversed rules;
+# Clang/MSVC keep pre-#235 behavior — see ninja_backend posixDepfile gate).
 # depfile. Before this fix, `cxx_module`/`cxx_object` had NO depfile on
 # non-MSVC (only the msvcDeps branch added `deps=msvc`) — the P1689 scan's
 # `$out.dep` was generated then discarded. So editing a file `#include`d