diff --git a/src/cpp/cmake/ScadCompile.cmake b/src/cpp/cmake/ScadCompile.cmake
index bf0c6470..f7a40f0b 100644
--- a/src/cpp/cmake/ScadCompile.cmake
+++ b/src/cpp/cmake/ScadCompile.cmake
@@ -31,6 +31,11 @@ function(scad_apply_compile_options target)
     target_compile_definitions(${target} PRIVATE
       $<$<CONFIG:Debug>:_GLIBCXX_ASSERTIONS>
       $<$<CONFIG:Debug>:_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_EXTENSIVE>)
+    # MinGW: OCCT headers (gp_Dir.hxx & co.) use M_PI, which MinGW's math.h hides under
+    # -std=c++20 (strict ANSI) unless _USE_MATH_DEFINES is set; MSVC already gets it above.
+    if(MINGW)
+      target_compile_definitions(${target} PRIVATE _USE_MATH_DEFINES)
+    endif()
     if(SCAD_SANITIZE)
       target_compile_options(${target} PRIVATE -fsanitize=address,undefined -fno-omit-frame-pointer)
       target_link_options(${target} PRIVATE -fsanitize=address,undefined)
diff --git a/src/cpp/tools/build_occt.py b/src/cpp/tools/build_occt.py
index 8cb5b5a8..c35252d5 100644
--- a/src/cpp/tools/build_occt.py
+++ b/src/cpp/tools/build_occt.py
@@ -119,10 +119,10 @@ def main() -> int:
         "INSTALL_DIR_LAYOUT": "Unix",
     }
     cmd = ["cmake", "-S", str(src), "-B", str(build)]
-    # Ninja where a compiler is already on the path (Linux, macOS).  On Windows only when the MSVC environment is active
-    # (`cl` on the path: the release workflow activates it); otherwise the default Visual Studio generator finds MSVC by
-    # itself, which a bare Ninja would not.
-    ninja = shutil.which("ninja") is not None and (sys.platform != "win32" or shutil.which("cl") is not None)
+    # Ninja where a compiler is already on the path (Linux, macOS).  On Windows when the MSVC environment is active
+    # (`cl` on the path: the release workflow activates it) or a MinGW g++ is (with cl absent CMake falls back to
+    # gcc); otherwise the default Visual Studio generator finds MSVC by itself, which a bare Ninja would not.
+    ninja = shutil.which("ninja") is not None and (sys.platform != "win32" or shutil.which("cl") is not None or shutil.which("g++") is not None)
     if ninja:
         cmd += ["-G", "Ninja"]
     cmd += [f"-D{k}={v}" for k, v in options.items()]