define default prefix constants

also use them to detect if prefix line in bottle block is needed

Library/Homebrew/dev-cmd/bottle.rb

@@ -16,7 +16,7 @@ BOTTLE_ERB = <<-EOS.freeze
     <% if !root_url.start_with?(HOMEBREW_BOTTLE_DEFAULT_DOMAIN) %>
     root_url "<%= root_url %>"
     <% end %>
-    <% if ![Homebrew::DEFAULT_PREFIX, "/usr/local"].include?(prefix) %>
+    <% if ![HOMEBREW_DEFAULT_PREFIX, LINUXBREW_DEFAULT_PREFIX].include?(prefix) %>
     prefix "<%= prefix %>"
     <% end %>
     <% if cellar.is_a? Symbol %>

Library/Homebrew/global.rb

@@ -52,6 +52,9 @@ HOMEBREW_USER_AGENT_FAKE_SAFARI =
 HOMEBREW_BOTTLE_DEFAULT_DOMAIN = ENV["HOMEBREW_BOTTLE_DEFAULT_DOMAIN"]
 HOMEBREW_BOTTLE_DOMAIN = ENV["HOMEBREW_BOTTLE_DOMAIN"]
 
+HOMEBREW_DEFAULT_PREFIX = "/usr/local"
+LINUXBREW_DEFAULT_PREFIX = "/home/linuxbrew/.linuxbrew"
+
 require "fileutils"
 require "os"
 require "os/global"
@@ -59,7 +62,7 @@ require "os/global"
 module Homebrew
   extend FileUtils
 
-  DEFAULT_PREFIX ||= "/usr/local"
+  DEFAULT_PREFIX ||= HOMEBREW_DEFAULT_PREFIX
   DEFAULT_CELLAR = "#{DEFAULT_PREFIX}/Cellar"
   DEFAULT_REPOSITORY = "#{DEFAULT_PREFIX}/Homebrew"
 

Library/Homebrew/os/linux/global.rb

@@ -2,8 +2,8 @@
 
 module Homebrew
   DEFAULT_PREFIX ||= if ENV["HOMEBREW_FORCE_HOMEBREW_ON_LINUX"]
-    "/usr/local"
+    HOMEBREW_DEFAULT_PREFIX
   else
-    "/home/linuxbrew/.linuxbrew"
+    LINUXBREW_DEFAULT_PREFIX
   end.freeze
 end