configure: Check for and explicitly link against -latomic

Some C libraries, such as uClibc, require an explicit link for some atomic
functions. Check for any libatomic, and explcily link it.

src/charon-cmd/Makefile.am

@@ -20,4 +20,4 @@ AM_CPPFLAGS = \
 charon_cmd_LDADD = \
 	$(top_builddir)/src/libstrongswan/libstrongswan.la \
 	$(top_builddir)/src/libcharon/libcharon.la \
-	-lm $(PTHREADLIB) $(DLLIB)
+	-lm $(PTHREADLIB) $(ATOMICLIB) $(DLLIB)