fix: use _NSGetExecutablePath on macOS (no /proc/self/exe)

macOS doesn't have /proc/self/exe. Use _NSGetExecutablePath from
mach-o/dyld.h to resolve the binary's absolute path. This fixes:
- build.ninja mcpp= variable (needed for dyndep)
- MCPP_HOME auto-detection from binary location

Author: Sunrisepeak

src/build/ninja_backend.cppm

@@ -14,6 +14,9 @@
 module;
 #include <cstdio>
 #include <cstdlib>
+#if defined(__APPLE__)
+#include <mach-o/dyld.h>  // _NSGetExecutablePath
+#endif
 
 export module mcpp.build.ninja;
 
@@ -113,9 +116,19 @@ bool dyndep_mode_enabled() {
 
 std::filesystem::path mcpp_exe_path() {
     std::error_code ec;
+#if defined(__APPLE__)
+    // macOS: use _NSGetExecutablePath
+    char buf[4096];
+    uint32_t size = sizeof(buf);
+    if (_NSGetExecutablePath(buf, &size) == 0) {
+        auto p = std::filesystem::canonical(buf, ec);
+        if (!ec) return p;
+    }
+#else
     auto p = std::filesystem::read_symlink("/proc/self/exe", ec);
     if (!ec)
         return p;
+#endif
     return "mcpp";  // fall back to PATH lookup
 }
 

src/config.cppm

@@ -16,6 +16,9 @@
 module;
 #include <cstdio>
 #include <cstdlib>
+#if defined(__APPLE__)
+#include <mach-o/dyld.h>  // _NSGetExecutablePath
+#endif
 
 export module mcpp.config;
 
@@ -161,7 +164,15 @@ std::filesystem::path home_dir() {
         return std::filesystem::path(e);
 
     std::error_code ec;
+#if defined(__APPLE__)
+    char _exe_buf[4096];
+    uint32_t _exe_size = sizeof(_exe_buf);
+    std::filesystem::path exe;
+    if (_NSGetExecutablePath(_exe_buf, &_exe_size) == 0)
+        exe = std::filesystem::canonical(_exe_buf, ec);
+#else
     auto exe = std::filesystem::canonical("/proc/self/exe", ec);
+#endif
     if (!ec && exe.parent_path().filename() == "bin") {
         // Dev builds emit binaries at target/<triple>/<fp>/bin/<exe>,
         // matching the bin/ shape. Any ancestor literally named