exit if needed libs not found

configure.ac

@@ -17,9 +17,12 @@ AC_PROG_LIBTOOL
 AM_PROG_CC_C_O
 
 # Checks for libraries.
-AC_CHECK_LIB([mcrypt], [mcrypt_module_open])
-AC_CHECK_LIB([mhash], [mhash_keygen])
-AC_CHECK_LIB([json], [json_object_new_object])
+AC_CHECK_LIB([mcrypt], [mcrypt_module_open], [],
+	     [AC_MSG_ERROR([libmcrypt not found], [1])])
+AC_CHECK_LIB([mhash], [mhash_keygen], [],
+	     [AC_MSG_ERROR([libmhash not found], [1])])
+AC_CHECK_LIB([json], [json_object_new_object], [],
+	     [AC_MSG_ERROR([json-c library not found], [1])])
 
 # Checks for header files.
 AC_CHECK_HEADERS([fcntl.h limits.h stddef.h stdlib.h string.h])