Simplify build script compile src/gethrtime.c for all platforms

Instead of having a lot of ifs in the Makefile.am etc
we can just compile the file for all platforms and protect
the content with an ifndef in the file

Change-Id: I03d9c77c39b4e29b4c83cf68911a0ace97d44c23
Reviewed-on: http://review.couchbase.org/26775
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>