[maint] Reworked C++03 solution for thread-local error

[ethouris]

Fixes #3311

The change:

1. Completely removed thread-local error facilities split between STDCXX_SYNC and POSIX_SYNC versions.
2. Added C++11 version and C++03/POSIX version for thread-local facility. The latter is in force only if compiling with enforced C++98 compatibility.
3. C++11 version: keep the thread-local error object as static thread_local inside the AccessThreadLocalError function. This function then guarantees always possible access to the object for setting and obtaining.
4. C++98 version: Use the CThreadLocal wrapper to ensure unique and per-thread object initialization:
   • The key is created during initialization of CThreadLocal-wrapped object declared as local static (initialized at the first call of AccessThreadLocalError) by pthread_key_create
   • Every call to AccessThreadLocalError uses CThreadLocal::get to get pointer to the object. The object is lazy-initialized. There exists a possibility that the allocation will fail and the pointer remains NULL. The successfully allocated object is saved by pthread_setspecific under the globally-accessible key so that further calls obtain it by pthread_getspecific.
   • The call to SetThreadLocalError gets first access to the object by get; if this fails, setting is silently ignored.
   • The call to GetThreadLocalError gets first access to the object by get; if this fails, the fallback error object (this time as simply global static object) is returned, preconfigured as memory allocation error
   • The call to ClearThreadLocalError sets the error as success, again, only if get returns the valid object and it's ignored otherwise
   • The destructor of CThreadLocal deletes also the object that is assigned in the current thread; it is believed that this will only happen in the same thread that will exit as the last one, so it's a safety precaution to not access it too late.

NOTE: A small change in the API was also necessary: the CUDT::getLastError now returns the error as constant reference. This shouldn't matter provided that this is an unofficial C++ API only and for the official C API this doesn't change anything. This is a safety precaution, which although the allocation error possible in the POSIX version is highly unlikely, the safety precaution in case when the fallback memory allocation resident error is returned as a fallback, it is not allowed to be modified.