Output more warnings on unsupported configurations

Library/Homebrew/diagnostic.rb

@@ -79,6 +79,10 @@ module Homebrew
         ].freeze
       end
 
+      def supported_configuration_checks
+        [].freeze
+      end
+
       def development_tools_checks
         %w[
           check_for_installed_developer_tools
@@ -97,11 +101,10 @@ module Homebrew
 
       def please_create_pull_requests(what = "unsupported configuration")
         <<~EOS
-          You may encounter build failures and other breakages.
-          Please create pull requests instead of asking for help on
-          Homebrew's GitHub, Discourse, Twitter or IRC. You are
-          responsible for resolving any issues you experience, as
-          you are running this #{what}.
+          You will encounter build failures with some formulae.
+          Please create pull requests instead of asking for help on Homebrew's GitHub,
+          Discourse, Twitter or IRC. You are responsible for resolving any issues you
+          experience, as you are running this #{what}.
         EOS
       end
 
@@ -118,10 +121,8 @@ module Homebrew
         return unless ENV["HOMEBREW_BUILD_FROM_SOURCE"]
 
         <<~EOS
-          You have HOMEBREW_BUILD_FROM_SOURCE set. This environment variable is
-          intended for use by Homebrew developers. If you are encountering errors,
-          please try unsetting this. Please do not file issues if you encounter
-          errors when using this environment variable.
+          You have HOMEBREW_BUILD_FROM_SOURCE set.
+          #{please_create_pull_requests}
         EOS
       end
 
@@ -806,9 +807,9 @@ module Homebrew
 
         <<~EOS
           Your Homebrew's prefix is not #{Homebrew::DEFAULT_PREFIX}.
-          You can install Homebrew anywhere you want but some bottles (binary packages)
-          can only be used with a standard prefix and some formulae (packages)
-          may not build correctly with a non-standard prefix.
+          Some of Homebrew's bottles (binary packages) can only be used with the default
+          prefix (#{Homebrew::DEFAULT_PREFIX}).
+          #{please_create_pull_requests}
         EOS
       end
 

Library/Homebrew/extend/os/linux/diagnostic.rb

@@ -7,6 +7,13 @@ require "os/linux/kernel"
 module Homebrew
   module Diagnostic
     class Checks
+      def supported_configuration_checks
+        %w[
+          check_glibc_minimum_version
+          check_kernel_minimum_version
+        ].freeze
+      end
+
       def check_tmpdir_sticky_bit
         message = generic_check_tmpdir_sticky_bit
         return if message.nil?

Library/Homebrew/extend/os/linux/install.rb

@@ -26,8 +26,8 @@ module Homebrew
       FileUtils.ln_sf ld_so, brew_ld_so
     end
 
-    def perform_preinstall_checks
-      generic_perform_preinstall_checks
+    def perform_preinstall_checks(all_fatal: false)
+      generic_perform_preinstall_checks(all_fatal: all_fatal)
       symlink_ld_so
     end
   end

Library/Homebrew/extend/os/mac/diagnostic.rb

@@ -1,12 +1,19 @@
 module Homebrew
   module Diagnostic
     class Checks
-      undef development_tools_checks, fatal_development_tools_checks,
-            build_error_checks
+      undef supported_configuration_checks, development_tools_checks,
+            fatal_development_tools_checks, build_error_checks
+
+      def supported_configuration_checks
+        %w[
+          check_build_from_source
+          check_homebrew_prefix
+          check_for_unsupported_macos
+        ].freeze
+      end
 
       def development_tools_checks
         %w[
-          check_for_unsupported_macos
           check_for_installed_developer_tools
           check_xcode_license_approved
           check_xcode_up_to_date

Library/Homebrew/install.rb

@@ -17,6 +17,16 @@ module Homebrew
       end
     end
 
+    def check_cc_argv
+      return unless ARGV.cc
+
+      @checks ||= Diagnostic::Checks.new
+      opoo <<~EOS
+        You passed `--cc=#{ARGV.cc}`.
+        #{@checks.please_create_pull_requests}
+      EOS
+    end
+
     def attempt_directory_creation
       Keg::MUST_EXIST_DIRECTORIES.each do |dir|
         begin
@@ -28,28 +38,34 @@ module Homebrew
     end
 
     def perform_development_tools_checks
-      fatal_checks(:fatal_development_tools_checks)
+      diagnostic_checks(:fatal_development_tools_checks)
     end
 
-    def perform_preinstall_checks
+    def perform_preinstall_checks(all_fatal: false)
       check_cpu
       attempt_directory_creation
-      fatal_checks(:fatal_install_checks)
+      check_cc_argv
+      diagnostic_checks(:supported_configuration_checks, fatal: all_fatal)
+      diagnostic_checks(:fatal_install_checks)
     end
     alias generic_perform_preinstall_checks perform_preinstall_checks
     module_function :generic_perform_preinstall_checks
 
-    def fatal_checks(type)
+    def diagnostic_checks(type, fatal: true)
       @checks ||= Diagnostic::Checks.new
       failed = false
       @checks.public_send(type).each do |check|
         out = @checks.public_send(check)
         next if out.nil?
 
-        failed ||= true
-        ofail out
+        if fatal
+          failed ||= true
+          ofail out
+        else
+          opoo out
+        end
       end
-      exit 1 if failed
+      exit 1 if failed && fatal
     end
   end
 end