https://github.com/miniupnp/libnatpmp/pull/39/commits/e654ac0638fd014d6279bafe646f049a95198661

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -57,7 +57,7 @@ install(TARGETS natpmp natpmpc
         LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
         ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})
 
-install(FILES natpmp.h
+install(FILES natpmp.h natpmp_declspec.h
         DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/natpmp.pc
