check for libsodium, libev, libudns if configured with system library

configure.ac

@@ -304,8 +304,11 @@ dnl Add define for libudns to enable IPv6 support
 dnl This is an option defined in the origin configure script
 AC_DEFINE([HAVE_IPv6], [1], [Enable IPv6 support in libudns])
 
-AM_COND_IF([USE_SYSTEM_SHARED_LIB],
-  [],
+AM_COND_IF([USE_SYSTEM_SHARED_LIB],[
+    AC_CHECK_LIB([sodium], [sodium_init], ,[
+       AC_MSG_ERROR([Couldn't find libsodium. Try installing libsodium-dev@<:@el@:>@.])
+    ])
+  ],
   [AC_CONFIG_SUBDIRS([libsodium])])
 
 AC_CONFIG_FILES([ shadowsocks-libev.pc
@@ -313,8 +316,10 @@ AC_CONFIG_FILES([ shadowsocks-libev.pc
                  libcork/Makefile
                  libipset/Makefile
                  src/Makefile])
-AM_COND_IF([USE_SYSTEM_SHARED_LIB],
-  [],
+AM_COND_IF([USE_SYSTEM_SHARED_LIB],[
+    AC_CHECK_LIB([udns], [dns_dnlen], ,[AC_MSG_ERROR([Couldn't find libudns. Try installing libudns-dev or udns-devel.])])
+    AC_CHECK_LIB([ev], [ev_loop_destroy], ,[AC_MSG_ERROR([Couldn't find libev. Try installing libev-dev@<:@el@:>@.])])
+  ],
   [AC_CONFIG_FILES([libudns/Makefile
                  libev/Makefile])])