diff --git a/implementations/Crafter.Build-External.cpp b/implementations/Crafter.Build-External.cpp
index 4a9fbe2..d209a00 100644
--- a/implementations/Crafter.Build-External.cpp
+++ b/implementations/Crafter.Build-External.cpp
@@ -179,6 +179,19 @@ std::string ConfigureCMake(const fs::path& cloneDir, const fs::path& cmakeBuildD
 
     if (!needsConfigure) return "";
 
+    // A build directory configured with other inputs starts over instead of
+    // being reconfigured. CMake reconfigures in place without recompiling
+    // what an old compiler or --sysroot compiled (the Makefiles generator
+    // writes --sysroot into build.make, which make does not compare) and
+    // keeps the check results the old configure cached. A mingw external
+    // built against the host's /usr as the sysroot (issue #43) kept its
+    // objects reading __libc_single_threaded after the fix that way. The
+    // directory keeps its name, which projects point generated headers at.
+    if (fs::exists(cmakeBuildDir / "CMakeCache.txt")) {
+        std::error_code ec;
+        fs::remove_all(cmakeBuildDir, ec);
+        if (ec) return std::format("cannot clear {} for a fresh configure: {}", cmakeBuildDir.string(), ec.message());
+    }
     if (!fs::exists(cmakeBuildDir)) fs::create_directories(cmakeBuildDir);
 
     std::string cmd = std::format("cmake -S {} -B {}{}", ShellQuote(fs::absolute(cloneDir).string()), ShellQuote(fs::absolute(cmakeBuildDir).string()), injected);