cmake: include -ldl via cmake built-in var

This does two things:
1. fixes clang build, which otherwise errors with undefined symbol
'dlsym'.
2. simplifies the cmake script, delegating to cmake to figure
   out platform-specific flags for linking against the dl library.

CMakeLists.txt

@@ -455,10 +455,7 @@ elseif(NOT MSVC)
   set(EXTRA_LIBRARIES ${RT})
 endif()
 
-if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT MINGW)
+list(APPEND EXTRA_LIBRARIES ${CMAKE_DL_LIBS})
-  find_library(DL dl)
-  set(EXTRA_LIBRARIES ${DL})
-endif()
 
 include(version.cmake)