Problem: building on macOS without --os flag

It'll fail because we can't autodetect the OS.

Solution: make macOS detectable but fail on anything but RedHat-based
(because that's the only systems we support right this moment)

exe/pgpm

@@ -85,6 +85,10 @@ module Pgpm
             exit(1)
           end
 
+          unless os.is_a?(Pgpm::OS::RedHat)
+            puts "#{os.name} is not a supported OS at this moment"
+            exit(1)
+          end
           puts "Building #{pkgs.map { |p| "#{p.name}@#{p.version}" }.join(", ")} for Postgres #{matching_pgver}"
           selected_pgdist = Postgres::RedhatBasedPgdg.new(matching_pgver.to_s)
 

lib/pgpm/os.rb

@@ -37,9 +37,12 @@ def with_scope(&block)
     end
 
     def self.auto_detect
-      return unless RUBY_PLATFORM =~ /linux$/
-
-      Pgpm::OS::Linux.auto_detect
+      if RUBY_PLATFORM =~ /linux$/
+        Pgpm::OS::Linux.auto_detect
+      else
+        RUBY_PLATFORM =~ /darwin/
+        Pgpm::OS::Darwin.auto_detect
+      end
     end
 
     def self.find(name)

lib/pgpm/os/darwin.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module Pgpm
+  module OS
+    class Darwin < Pgpm::OS::Unix
+      def self.name
+        "darwin"
+      end
+
+      def self.auto_detect
+        new
+      end
+    end
+  end
+end