fix(big-sur): added support for building on Big Sur

Curtsy of https://apple.stackexchange.com/questions/408999/gfortran-compiler-error-on-mac-os-big-sur

build-emacs-for-macos

@@ -298,6 +298,11 @@ class Build
         "#{brew_dir}/opt/texinfo/bin",
         ENV['PATH']
       ].compact.join(':')
+      
+      ENV['LIBRARY_PATH'] = [
+         ENV['LIBRARY_PATH'],
+         "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib"
+        ].compact.join(':')
 
       configure_flags = [
         '--with-ns',