Add a rdrand plugin stub detecting availability of RDRAND instructions

src/libstrongswan/plugins/rdrand/Makefile.am

@@ -0,0 +1,15 @@
+
+INCLUDES = -I$(top_srcdir)/src/libstrongswan
+
+AM_CFLAGS = -rdynamic
+
+if MONOLITHIC
+noinst_LTLIBRARIES = libstrongswan-rdrand.la
+else
+plugin_LTLIBRARIES = libstrongswan-rdrand.la
+endif
+
+libstrongswan_rdrand_la_SOURCES = \
+	rdrand_plugin.h rdrand_plugin.c
+
+libstrongswan_rdrand_la_LDFLAGS = -module -avoid-version