Allow custom LDFLAGS
Allow custom LDFLAGS

- Allow end user to specify LDFLAGS via environment variables
- Remove spaces and use tabs everywhere
- Make the Makefile more verbose to have detailes logs in CI/CD tools
- Make the Makefile aware of its own filename which may differ from
  Makefile
- Fix `rm` bug in Windows

CAT: #refactor

eawsy/aws-lambda-go-shim,eawsy/aws-lambda-go-shim,eawsy/aws-lambda-go-shim,eawsy/aws-lambda-go-shim,eawsy/aws-lambda-go-shim,eawsy/aws-lambda-go-shim,eawsy/aws-lambda-go-shim