issue building curve-keygen

[diehard2]

Having issues with paths and static libs. The following fixes it

if (ENABLE_CURVE)
  add_executable(curve_keygen tools/curve_keygen.cpp)
  target_include_directories(curve_keygen PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>)
  if(BUILD_STATIC)
    target_link_libraries(curve_keygen libzmq-static)
  else()
    target_link_libraries(curve_keygen libzmq)
  endif()
  install(TARGETS curve_keygen RUNTIME DESTINATION bin)
endif()