fix: add popen/_popen compat to remaining Windows-affected files

publisher.cppm, stdmod.cppm, p1689.cppm, ninja_backend.cppm all
use popen/pclose which is _popen/_pclose on Windows MSVC.

Author: Sunrisepeak

src/build/ninja_backend.cppm

@@ -16,6 +16,8 @@ module;
 #include <cstdlib>
 #if defined(_WIN32)
 #include <windows.h>
+#define popen  _popen
+#define pclose _pclose
 #elif defined(__APPLE__)
 #include <mach-o/dyld.h>  // _NSGetExecutablePath
 #endif

src/modgraph/p1689.cppm

@@ -19,6 +19,10 @@
 module;
 #include <cstdio>
 #include <cstdlib>
+#if defined(_WIN32)
+#define popen  _popen
+#define pclose _pclose
+#endif
 
 export module mcpp.modgraph.p1689;
 

src/pm/publisher.cppm

@@ -4,6 +4,10 @@
 
 module;
 #include <cstdio>     // popen / pclose / fgets
+#if defined(_WIN32)
+#define popen  _popen
+#define pclose _pclose
+#endif
 
 export module mcpp.pm.publisher;
 

src/toolchain/stdmod.cppm

@@ -1,6 +1,10 @@
 module;
 #include <cstdio>     // popen, pclose, fgets, FILE
 #include <cstdlib>    // getenv
+#if defined(_WIN32)
+#define popen  _popen
+#define pclose _pclose
+#endif
 
 // mcpp.toolchain.stdmod — pre-build the `import std` BMI and cache it.
 //