fix(flags): force winpthread static on MinGW links

winlibs POSIX-threads libstdc++ otherwise leaves produced exes depending on
libwinpthread-1.dll — the standalone-exe e2e caught it (empty output,
missing-DLL abort). -Wl,-Bstatic -lwinpthread -Wl,-Bdynamic is the standard
recipe; libwinpthread.a ships in the payload.

Author: Sunrisepeak

src/build/flags.cppm

@@ -350,8 +350,11 @@ CompileFlags compute_flags(const BuildPlan& plan) {
             if (f.linkage == "static") mingw_static = " -static";
             // std::print's terminal probe (__open_terminal /
             // __write_to_terminal, bits/print.h) lives in libstdc++exp.a on
-            // Windows targets — plain -lstdc++ leaves them undefined.
-            mingw_stdexp = " -lstdc++exp";
+            // Windows targets — plain -lstdc++ leaves them undefined. And
+            // POSIX-threads libstdc++ (winlibs) pulls libwinpthread-1.dll
+            // unless winpthread is forced static — the classic third DLL
+            // that -static-libstdc++/-static-libgcc do NOT cover.
+            mingw_stdexp = " -lstdc++exp -Wl,-Bstatic -lwinpthread -Wl,-Bdynamic";
         }
         f.ld = std::format("{}{}{}{}{}{}", mingw_static, static_stdlib, b_flag,
                            user_ldflags, mingw_stdexp, link_extra);