Keep ABI compatible with already compiled LGPL applications

Applications with _LGPL_SOURCE defined that were compiled against bogus
tls-compat.h header *and* which are using multiple urcu flavors
concurrently need to be told that they need to be recompiled against a
fixed tls-compat.h header. Detect these usage, and abort() with a
message error on stderr.

This is needed for stable-0.7 and stable-0.8 branches of userspace RCU
only. The upcoming 0.9 will bump the soname version, and therefore does
not have to care about this aspect of ABI compatibility.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>