error.h: Never call (para_)strerror() on osl errors.

error.h: Never call (para_)strerror() on osl errors.

In the (inlined) function para_strerror() of error.h we compile in
the call to osl_strerror() if and only if osl.h has been included
(in which case the preprocessor macro _OSL_H is defined). This is
wrong because an osl error code might well be passed to a function
defined in a compilation unit which does not include osl.h. If this
function calls para_strerror(), we will segfault or worse.

We need to check at link time whether osl_strerror is a defined
symbol rather than checking a preprocessor macro at compile time.
Fortunately, a little linker fu helps us out here. This patch
introduces a weak reference to osl_strerror to tell whether
osl_strerror is defined.