Fix CMake ENABLE_COROUTINES flag with Boost < 1.59 (#335)

Co-authored-by: Cam Fulton <[email protected]>

Author: fultoncjb

CMakeLists.txt

@@ -48,6 +48,9 @@ if(ENABLE_COROUTINES)
     endif()
 else()
     message(STATUS "Boost coroutines disabled by CMake option.")
+endif()
+
+if(NOT DEFINED BT_COROUTINES)
     add_definitions(-DBT_NO_COROUTINES)
 endif()