[R-SIG-Mac] "cannot run mixed C/Fortran code"

[Duncan Murdoch]

After updating to Catalina, I've installed the Xcode command line tools 
version 11.4.1, and gfortran 8.2.  When trying to build R, configure dies:

  checking whether gfortran appends underscores to external names... yes
  checking whether gfortran appends extra underscores to external 
names... no
  checking whether mixed C/Fortran code can be run... configure: 
WARNING: cannot run mixed C/Fortran code
  configure: error: Maybe check LDFLAGS for paths to Fortran libraries?

I don't have LDFLAGS set.  I'm not certain I'm running the right 
compilers (I might have old ones that have precedence on the path).  Are 
these the versions I should be seeing?

$ clang --version
Apple clang version 11.0.3 (clang-1103.0.32.59)
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

$ gfortran --version
GNU Fortran (GCC) 8.2.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

If those are the right versions, any other suggestions on how to fix the 
configure problem?

Duncan Murdoch