add EmacsRequirement

Closes Homebrew/homebrew#39326.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/dependency_collector.rb

@@ -121,6 +121,7 @@ class DependencyCollector
     when :tuntap     then TuntapDependency.new(tags)
     when :ant        then ant_dep(spec, tags)
     when :apr        then AprDependency.new(tags)
+    when :emacs      then EmacsRequirement.new(tags)
     # Tiger's ld is too old to properly link some software
     when :ld64       then LD64Dependency.new if MacOS.version < :leopard
     when :clt # deprecated

Library/Homebrew/requirements.rb

@@ -12,6 +12,7 @@ require 'requirements/ruby_requirement'
 require 'requirements/tuntap_dependency'
 require 'requirements/unsigned_kext_requirement'
 require 'requirements/x11_dependency'
+require 'requirements/emacs_requirement'
 
 class XcodeDependency < Requirement
   fatal true

Library/Homebrew/requirements/emacs_requirement.rb

@@ -0,0 +1,30 @@
+class EmacsRequirement < Requirement
+  fatal true
+  default_formula "emacs"
+
+  def initialize(tags)
+    @version = tags.shift if /\d+\.*\d*/ === tags.first
+    raise "Specify a version for EmacsRequirement" unless @version
+    super
+  end
+
+  satisfy :build_env => false do
+    next unless which "emacs"
+    emacs_version = Utils.popen_read("emacs", "--batch", "--eval", "(princ emacs-version)")
+    Version.new(emacs_version) >= Version.new(@version)
+  end
+
+  env do
+    ENV.prepend_path "PATH", which("emacs").dirname
+  end
+
+  def message
+    s = "Emacs #{@version} or later is required."
+    s += super
+    s
+  end
+
+  def inspect
+    "#<#{self.class.name}: #{name.inspect} #{tags.inspect} version=#{@version.inspect}>"
+  end
+end