ignore feature using syscall for appengine users

[tutuming]

for #38

go-metrics is imported from goa ( https://github.com/goadesign/goa ) and some users (including me) use appengine to serve APIs.

Build for appengine failed because import of syscall package is not allowed.
So for the appengine users, ignoring the InmemSignal feature make them happy.

[pei0804]

I hope this request will be merged soon

[ryanuber]

This seems fine to me. Can you also add a note to the README that the InmemSignal is not supported on this platform?

[pei0804]

Go-app-builder: Failed parsing input: parser: bad import "syscall" in ...
This problem is a problem occurring in GAE which is PaaS of GCP.
Two packages, syscall and unsafe, are not allowed to be used on GAE.
So, you need a description not included in the build.

The target files are as follows.

• const_unix.go
• const_windows.go
• inmem_signal.go
• inmem_signal_test.go

We have created a request.
Verification please.
#47

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