Adjusted pthread LDFLAG in examples required for multithreading.

- Some versions of gcc/libc require setting the pthread flag when using
  std::thread, which to implement it.
- This was found compiling the library in a Debian (bullseye) container.

examples/reading_logs_via_rule_message/Makefile.am

@@ -21,6 +21,7 @@ simple_request_LDFLAGS = \
 	-L$(top_builddir)/src/.libs/ \
 	$(GEOIP_LDFLAGS) \
 	-lmodsecurity \
+	-lpthread \
 	-lm \
 	-lstdc++ \
 	$(LMDB_LDFLAGS) \