Add depends_on :linux and :macos

Library/Homebrew/dependency_collector.rb

@@ -105,6 +105,7 @@ class DependencyCollector
     case spec
     when :x11        then X11Requirement.new(spec.to_s, tags)
     when :xcode      then XcodeRequirement.new(tags)
+    when :linux      then LinuxRequirement.new(tags)
     when :macos      then MinimumMacOSRequirement.new(tags)
     when :mysql      then MysqlRequirement.new(tags)
     when :postgresql then PostgresqlRequirement.new(tags)

Library/Homebrew/requirements.rb

@@ -2,6 +2,7 @@ require "requirement"
 require "requirements/fortran_requirement"
 require "requirements/gpg2_requirement"
 require "requirements/language_module_requirement"
+require "requirements/linux_requirement"
 require "requirements/minimum_macos_requirement"
 require "requirements/maximum_macos_requirement"
 require "requirements/mpi_requirement"

Library/Homebrew/requirements/linux_requirement.rb

+class LinuxRequirement < Requirement
+  fatal true
+
+  satisfy(build_env: false) { OS.linux? }
+
+  def message
+    "Linux is required."
+  end
+end

Library/Homebrew/requirements/minimum_macos_requirement.rb

@@ -3,18 +3,29 @@ require "requirement"
 class MinimumMacOSRequirement < Requirement
   fatal true
 
-  def initialize(tags)
-    @version = MacOS::Version.from_symbol(tags.first)
+  def initialize(tags = [])
+    @version = MacOS::Version.from_symbol(tags.first) unless tags.empty?
     super
   end
 
-  satisfy(build_env: false) { MacOS.version >= @version }
+  def minimum_version_specified?
+    OS.mac? && @version
+  end
+
+  satisfy(build_env: false) do
+    next MacOS.version >= @version if minimum_version_specified?
+    next true if OS.mac?
+    next true if @version
+    false
+  end
 
   def message
+    return "macOS is required." unless minimum_version_specified?
     "macOS #{@version.pretty_name} or newer is required."
   end
 
   def display_s
+    return "macOS is required" unless minimum_version_specified?
     "macOS >= #{@version}"
   end
 end

Library/Homebrew/test/os_requirement_spec.rb

+require "requirements/linux_requirement"
+require "requirements/minimum_macos_requirement"
+
+describe LinuxRequirement do
+  describe "#satisfied?" do
+    it "returns true if OS is Linux" do
+      expect(subject.satisfied?).to eq(OS.linux?)
+    end
+  end
+end
+
+describe MinimumMacOSRequirement do
+  describe "#satisfied?" do
+    it "returns true if OS is macOS" do
+      expect(subject.satisfied?).to eq(OS.mac?)
+    end
+  end
+end