ext-auth: Add an ext-auth plugin invoking an external authorization script

Original patch courtesy of Vyronas Tsingaras.

src/libcharon/plugins/ext_auth/Makefile.am

@@ -0,0 +1,18 @@
+AM_CPPFLAGS = \
+	-I$(top_srcdir)/src/libstrongswan \
+	-I$(top_srcdir)/src/libhydra \
+	-I$(top_srcdir)/src/libcharon
+
+AM_CFLAGS = \
+	$(PLUGIN_CFLAGS)
+
+if MONOLITHIC
+noinst_LTLIBRARIES = libstrongswan-ext-auth.la
+else
+plugin_LTLIBRARIES = libstrongswan-ext-auth.la
+endif
+
+libstrongswan_ext_auth_la_SOURCES = ext_auth_plugin.h ext_auth_plugin.c \
+	ext_auth_listener.h ext_auth_listener.c
+
+libstrongswan_ext_auth_la_LDFLAGS = -module -avoid-version