Fix copying 64 bit OpenAL DLL

ARCH_64BITS is defined within SFML but not visible to downstream
projects which meant this conditional always evaluated to false
and thus the 32 bit OpenAL DLL was used.

Author: ChrisThrasher

CMakeLists.txt

@@ -17,7 +17,7 @@ if(WIN32)
     add_custom_command(
         TARGET CMakeSFMLProject
         COMMENT "Copy OpenAL DLL"
-        PRE_BUILD COMMAND ${CMAKE_COMMAND} -E copy ${SFML_SOURCE_DIR}/extlibs/bin/$<IF:$<BOOL:${ARCH_64BITS}>,x64,x86>/openal32.dll $<TARGET_FILE_DIR:CMakeSFMLProject>
+        PRE_BUILD COMMAND ${CMAKE_COMMAND} -E copy ${SFML_SOURCE_DIR}/extlibs/bin/$<IF:$<EQUAL:${CMAKE_SIZEOF_VOID_P},8>,x64,x86>/openal32.dll $<TARGET_FILE_DIR:CMakeSFMLProject>
         VERBATIM)
 endif()