Make the target OS explicitly default to the host OS

This triggers the check for building on Windows to make binaries get the
".exe" extension.

appveyor.yml

@@ -4,9 +4,9 @@ install:
 
 build_script:
   - bash -c 'GOARCH=386 script/bootstrap'
-  - mv bin\git-lfs git-lfs-x86.exe
+  - mv bin\git-lfs.exe git-lfs-x86.exe
   - bash -c 'GOARCH=amd64 script/bootstrap'
-  - mv bin\git-lfs git-lfs-x64.exe
+  - mv bin\git-lfs.exe git-lfs-x64.exe
 
 after_build:
   - iscc script\windows-installer\inno-setup-git-lfs-installer.iss

script/build.go

@@ -16,7 +16,7 @@ import (
 )
 
 var (
-	BuildOS    = flag.String("os", "", "OS to target: darwin, freebsd, linux, windows")
+	BuildOS    = flag.String("os", runtime.GOOS, "OS to target: darwin, freebsd, linux, windows")
 	BuildArch  = flag.String("arch", "", "Arch to target: 386, amd64")
 	BuildAll   = flag.Bool("all", false, "Builds all architectures")
 	ShowHelp   = flag.Bool("help", false, "Shows help")