Due to the setup of gb_DEBUGLEVEL in gbuild.mk, gb_SYMBOL was always
enabled when --enable-dbgutil is set, with no way to override it.
Fix that by turning configure's ENABLE_SYMBOLS into a tri-state, where
the new "FALSE" value, set by an explicit --disable-symbols, overrides
any implicit way of enabling symbols.
But by default an --enable-dbgutil still enables gb_SYMBOL.

Change-Id: I94c609863980ed1ab9c73d7a4861c394442b531d