RTS fails to build with --enable-debug

[catamorphism]

On my Mac, I can reproduce this with:

~/rust/build$ ../configure --enable-debug
~/rust/build$ make check

This yields:

compile: rt/x86_64-apple-darwin/rust_builtin.o
/Users/tchevalier/rust/src/rt/sync/lock_and_signal.h: In function 'void rust_wait_cond_lock(rust_cond_lock*)':
/Users/tchevalier/rust/src/rt/sync/lock_and_signal.h:27: error: 'bool lock_and_signal::lock_held_by_current_thread()' is private
/Users/tchevalier/rust/src/rt/rust_builtin.cpp:893: error: within this context
/Users/tchevalier/rust/src/rt/sync/lock_and_signal.h: In function 'bool rust_signal_cond_lock(rust_cond_lock*)':
/Users/tchevalier/rust/src/rt/sync/lock_and_signal.h:27: error: 'bool lock_and_signal::lock_held_by_current_thread()' is private
/Users/tchevalier/rust/src/rt/rust_builtin.cpp:907: error: within this context

[eholk]

Oops, this is my fault.