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/starter/Makefile.am

@@ -34,7 +34,7 @@ starter_LDADD = \
 	$(top_builddir)/src/libstrongswan/libstrongswan.la \
 	$(top_builddir)/src/libcharon/libcharon.la \
 	libstarter.la \
-	$(SOCKLIB) $(PTHREADLIB)
+	$(SOCKLIB) $(PTHREADLIB) $(ATOMICLIB)
 
 EXTRA_DIST = keywords.txt ipsec.conf Android.mk
 MAINTAINERCLEANFILES = keywords.c