windows: fix mingw build

src/CMakeLists.txt

 target_link_libraries(
   espeak-ng-bin PRIVATE espeak-ng espeak-ng-config
 )
+if (MINGW)
+  target_link_options(espeak-ng-bin PRIVATE "-static-libstdc++" "-static")
+endif()
 if (MSVC)
   target_sources(espeak-ng-bin PRIVATE compat/getopt.c)
 endif()

src/libespeak-ng/CMakeLists.txt

   )
 endif()
 
+if (MINGW)
+  target_link_options(espeak-ng PRIVATE "-static-libstdc++" "-static")
+endif()
+
 install(TARGETS espeak-ng LIBRARY)

tests/CMakeLists.txt

     $<TARGET_PROPERTY:espeak-ng,INTERFACE_INCLUDE_DIRECTORIES>
     $<TARGET_PROPERTY:espeak-ng-config,INTERFACE_INCLUDE_DIRECTORIES>
   )
+  if (MINGW)
+    target_link_options(test_${_test_name} PUBLIC "-static" "-static-libstdc++")
+  endif()
   add_dependencies(test_${_test_name} data)
   add_test(
     NAME ${_test_name}