Add :fortran requirement

Library/Homebrew/dependency_collector.rb

@@ -93,6 +93,7 @@ class DependencyCollector
     when :macos      then MinimumMacOSRequirement.new(tags)
     when :mysql      then MysqlDependency.new(tags)
     when :postgresql then PostgresqlDependency.new(tags)
+    when :fortran    then FortranRequired.new(tags)
     when :tex        then TeXDependency.new(tags)
     when :clt        then CLTDependency.new(tags)
     when :arch       then ArchRequirement.new(tags)

Library/Homebrew/requirements.rb

@@ -1,9 +1,10 @@
 require 'requirement'
+require 'requirements/fortran_dependency'
 require 'requirements/language_module_dependency'
-require 'requirements/x11_dependency'
+require 'requirements/macos_requirement'
 require 'requirements/mpi_dependency'
 require 'requirements/python_dependency'
-require 'requirements/macos_requirement'
+require 'requirements/x11_dependency'
 
 class XcodeDependency < Requirement
   fatal true

Library/Homebrew/requirements/fortran_dependency.rb

@@ -0,0 +1,24 @@
+require 'requirement'
+
+class FortranRequired < Requirement
+  fatal true
+
+  default_formula 'gfortran'
+
+  env { ENV.fortran }
+
+  satisfy :build_env => false do
+    (ENV['FC'] || which('gfortran')) ? true : false
+  end
+
+  def message; <<-EOS.undent
+    Fortran is required to install.
+
+    You can install this with Homebrew using:
+      brew install gfortran
+
+    Or you can use an external compiler by setting:
+      FC=<path-to-fortran-compiler>
+    EOS
+  end
+end