Add Fortran configuration to ENV.rb

This provides a uniform way for formulae to indicate a Fortran compiler
is required.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/extend/ENV.rb

@@ -131,6 +131,54 @@ module HomebrewEnvExtension
     self.O4
   end
 
+  def fortran
+    if self['FC']
+      ohai "Building with an alternative Fortran compiler. This is unsupported."
+      self['F77'] = self['FC'] unless self['F77']
+
+      if ARGV.include? '--default-fortran-flags'
+        self['FCFLAGS'] = self['CFLAGS'] unless self['FCFLAGS']
+        self['FFFLAGS'] = self['CFLAGS'] unless self['FFFLAGS']
+      elsif not self['FCFLAGS'] or self['FFLAGS']
+        opoo <<-EOS
+No Fortran optimization information was provided.  You may want to consider
+setting FCFLAGS and FFLAGS or pass the `--default-fortran-flags` option to
+`brew install` if your compiler is compatible with GCC.
+
+If you like the default optimization level of your compiler, ignore this
+warning.
+        EOS
+      end
+
+    elsif `/usr/bin/which gfortran`.chomp.size > 0
+      ohai <<-EOS
+Using Homebrew-provided fortran compiler.
+    This may be changed by setting the FC environment variable.
+      EOS
+      self['FC'] = `/usr/bin/which gfortran`.chomp
+      self['F77'] = self['FC']
+
+      self['FCFLAGS'] = self['CFLAGS']
+      self['FFLAGS'] = self['CFLAGS']
+
+    else
+      onoe <<-EOS
+This formula requires a fortran compiler, but we could not find one by
+looking at the FC environment variable or searching your PATH for `gfortran`.
+Please take one of the following actions:
+
+  - Decide to use the build of gfortran 4.2.x provided by Homebrew using
+        `brew install gfortran`
+
+  - Choose another Fortran compiler by setting the FC environment variable:
+        export FC=/path/to/some/fortran/compiler
+    Using an alternative compiler may produce more efficient code, but we will
+    not be able to provide support for build errors.
+      EOS
+      exit 1
+    end
+  end
+
   def osx_10_4
     self['MACOSX_DEPLOYMENT_TARGET']="10.4"
     remove_from_cflags(/ ?-mmacosx-version-min=10\.\d/)