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/libstrongswan/Makefile.am

@@ -120,7 +120,9 @@ endif
 
 library.lo :	$(top_builddir)/config.status
 
-libstrongswan_la_LIBADD = $(DLLIB) $(BTLIB) $(SOCKLIB) $(RTLIB) $(BFDLIB) $(UNWINDLIB)
+libstrongswan_la_LIBADD = \
+	$(DLLIB) $(ATOMICLIB)  $(BTLIB) $(SOCKLIB) \
+	$(RTLIB) $(BFDLIB) $(UNWINDLIB)
 
 AM_CPPFLAGS = \
 	-I$(top_srcdir)/src/libstrongswan \

src/libstrongswan/tests/Makefile.am

@@ -12,7 +12,7 @@ libtest_la_CFLAGS = \
 libtest_la_LDFLAGS = @COVERAGE_LDFLAGS@
 libtest_la_LIBADD = \
   $(top_builddir)/src/libstrongswan/libstrongswan.la \
-  $(PTHREADLIB)
+  $(PTHREADLIB) $(ATOMICLIB)
 
 
 TESTS = tests