[CMake] How to generate a STATIC Library using ExternalProject_Add

[Marshall, Rob]

Hi,

I am using ExternalProject_Add to build websocketpp.
The build is successful and libwebsocketpp.a is generated.
But it appears to be generated as a "UTILITY" rather than a "STATIC".

How can I configure ExternalProject_Add to build websocketapp as "STATIC"?

When I try and link in websocketpp with MyApp an error occurs:

CMake Error at CMake/CMakeMacros.cmake:434 (target_link_libraries):
  Target "websocketpp" of type UTILITY may not be linked into another target.
  One may link only to STATIC or SHARED libraries, or to executables with the
  ENABLE_EXPORTS property set.
Call Stack (most recent call first):
  /app/MyApp/CMakeLists.txt:25 (my_link_libraries)

Thanks,
Rob
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