Fix a build error on linux platforms

Due to dl* function usage in Core/HW/atrac3plus.cpp on non-windows
platforms, Core library needs to be linked with -ldl flag on these
platforms.

Added a CoreExtraLibs variable in CMakeLists.txt to handle all
platform-dependent dependencies of Core, and added dl to this variable
on non-win32 platforms.
Use CMAKE_DL_LIBS instead of dl
Modified the dl link flag to linux platform only
Merge pull request #2124 from chaserhkj/linux-fix

Fixed a build error on non-win32 platfroms