BUILD: tools: silence a build warning when USE_THREAD=0

BUILD: tools: silence a build warning when USE_THREAD=0

The dladdr_lock that was added to avoid re-entering into dladdr is
conditioned by threads, but the way it's declared causes a build
warning if threads are disabled due to the insertion of a lone semi
colon in the variables block. Let's switch to __decl_thread_var()
for this.

This can be backported wherever commit eb41d768f9 ("MINOR: tools:
use only opportunistic symbols resolution") is backported. It relies
on these previous two commits:

   bb4addabb7 ("MINOR: compiler: add a simple macro to concatenate resolved strings")
   69ac4cd315 ("MINOR: compiler: add a new __decl_thread_var() macro to declare local variables")

(cherry picked from commit b61ed9babe879fd5603af4643c1d60c8ab48d096)
Signed-off-by: Willy Tarreau <w@1wt.eu>