cmake: add curve_keygen binary

When sodium is enabled, also build curve_keygen binary. This is to
bring cmake builds to parity with autoconf.

Fixes: #4675

Author: xnox

CMakeLists.txt

@@ -1915,3 +1915,9 @@ if(ENABLE_NO_EXPORT)
   message(STATUS "Building with empty ZMQ_EXPORT macro")
   add_definitions(-DZMQ_NO_EXPORT)
 endif()
+
+if (ENABLE_CURVE)
+  add_executable(curve_keygen tools/curve_keygen.cpp)
+  target_link_libraries(curve_keygen libzmq)
+  install(TARGETS curve_keygen RUNTIME DESTINATION bin)
+endif()