add GSL as a development dependency

cmake/3rdPartyTargets.cmake

@@ -38,6 +38,10 @@ ADD_LIBRARY(cppzmq INTERFACE)
 TARGET_INCLUDE_DIRECTORIES(cppzmq INTERFACE "${CMAKE_SOURCE_DIR}/external/include")
 ADD_DEPENDENCIES(cppzmq libzmq)
 
+# define the import target of GSL
+ADD_LIBRARY(GSL INTERFACE)
+TARGET_INCLUDE_DIRECTORIES(GSL INTERFACE "${CMAKE_SOURCE_DIR}/external/include")
+
 # define the import target of protobuf
 ADD_LIBRARY(protobuf STATIC IMPORTED)
 IF (MSVC)