Unbreak the build by re-enabling exceptions.

Disabling them breaks build on archs using GCC. The problem is at line 156 of
bits/basic_ios.h:

	if (this->exceptions() & __state)
		__throw_exception_again;

With exceptions disabled __throw_exception_again is defined as

#define __throw_exception_again

at line 45 of exception_defines.h and the code results in an empty loop body,
which fails because of -Werror.

Approved by:	cognet