The following files in the hotspot source define the __USE_LEGACY_PROTOTYPES__ macro when it isn't required:

src/os/solaris/vm/jvm_solaris.h
src/share/vm/utilities/globalDefinitions_sparcWorks.hpp
hotspot/src/closed/os/solaris/vm/os_perf_solaris.cpp

In addition "__" macros are usually private to the system headers and may change at any time. The use of __USE_LEGACY_PROTOTYPES__ should therefore be removed.