Add HOMEBREW_FORCE_BOTTLE variable

This does the equivalent of always passing `--force-bottle`. This will
be enabled by default on Mojave to allow people to avoid building
everything from source until our porting and bottling is a bit further
along.

Library/Homebrew/brew.sh

@@ -262,6 +262,14 @@ then
 
   # Don't allow non-developers to customise Ruby warnings.
   unset HOMEBREW_RUBY_WARNINGS
+
+  # Default non-developers to bottles on prerelease versions of macOS
+  # in default prefix.
+  if [[ "$HOMEBREW_PREFIX" = "/usr/local" &&
+        "$HOMEBREW_MACOS_VERSION_NUMERIC" -ge "101400" ]]
+  then
+    export HOMEBREW_FORCE_BOTTLE="1"
+  fi
 fi
 
 if [[ -z "$HOMEBREW_RUBY_WARNINGS" ]]

Library/Homebrew/extend/ARGV.rb

@@ -262,7 +262,8 @@ module HomebrewArgvExtension
   end
 
   def force_bottle?
-    include? "--force-bottle"
+    return false if ENV["HOMEBREW_NO_FORCE_BOTTLE"]
+    include?("--force-bottle") || !ENV["HOMEBREW_FORCE_BOTTLE"].nil?
   end
 
   def fetch_head?

Library/Homebrew/manpages/brew.1.md.erb

@@ -170,6 +170,12 @@ Note that environment variables must have a value set to be detected. For exampl
     directories. TextMate can handle this correctly in project mode, but many
     editors will do strange things in this case.
 
+  * `HOMEBREW_FORCE_BOTTLE`:
+    If set, Homebrew will install from a bottle if it exists for the
+    current or newest version of macOS, even if it would not normally be used
+    for installation. Please do not file issues if you encounter errors when
+    using this environment variable.
+
   * `HOMEBREW_FORCE_BREWED_CURL`:
     If set, Homebrew will use a Homebrew-installed `curl` rather than the
     system version.

docs/Manpage.md

@@ -1167,6 +1167,12 @@ Note that environment variables must have a value set to be detected. For exampl
     directories. TextMate can handle this correctly in project mode, but many
     editors will do strange things in this case.
 
+  * `HOMEBREW_FORCE_BOTTLE`:
+    If set, Homebrew will install from a bottle if it exists for the
+    current or newest version of macOS, even if it would not normally be used
+    for installation. Please do not file issues if you encounter errors when
+    using this environment variable.
+
   * `HOMEBREW_FORCE_BREWED_CURL`:
     If set, Homebrew will use a Homebrew-installed `curl` rather than the
     system version.

manpages/brew.1

@@ -1109,6 +1109,10 @@ If set, Homebrew will use this editor when editing a single formula, or several
 \fINote:\fR \fBbrew edit\fR will open all of Homebrew as discontinuous files and directories\. TextMate can handle this correctly in project mode, but many editors will do strange things in this case\.
 .
 .TP
+\fBHOMEBREW_FORCE_BOTTLE\fR
+If set, Homebrew will install from a bottle if it exists for the current or newest version of macOS, even if it would not normally be used for installation\. Please do not file issues if you encounter errors when using this environment variable\.
+.
+.TP
 \fBHOMEBREW_FORCE_BREWED_CURL\fR
 If set, Homebrew will use a Homebrew\-installed \fBcurl\fR rather than the system version\.
 .