build: disable -Wself-assign

Prevent these warnings in clang 3.6:

    ./serialize.h:96:9: warning: explicitly assigning value of variable of type 'uint64_t' (aka 'unsigned long') to itself [-Wself-assign]
        obj = (obj);
        ~~~ ^  ~~~

configure.ac

@@ -160,7 +160,7 @@ fi
 ##       compatibility with the legacy buildsystem.
 ##
 if test "x$CXXFLAGS_overridden" = "xno"; then
-  CXXFLAGS="$CXXFLAGS -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameter"
+  CXXFLAGS="$CXXFLAGS -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameter -Wno-self-assign"
 fi
 CPPFLAGS="$CPPFLAGS -DBOOST_SPIRIT_THREADSAFE -DHAVE_BUILD_INFO -D__STDC_FORMAT_MACROS"