build-style/go: add support for build tags

Manual.md

@@ -1077,6 +1077,8 @@ The following variables influence how Go packages are built:
   will be downloaded with `go get`. Otherwise, a distfile has to be
   provided. This option should only be used with `-git` (or similar)
   packages; using a versioned distfile is preferred.
+- `go_build_tags`: An optional, space-separated list of build tags to
+  pass to Go.
 
 <a id="pkgs_haskell"></a>
 ### Haskell packages

common/build-style/go.sh

@@ -10,7 +10,7 @@ do_build() {
 	fi
 
 	go_package=${go_package:-$go_import_path}
-	go get -x ${go_package}
+	go get -x -tags "${go_build_tags}" ${go_package}
 }
 
 do_install() {