libSystem not found using gfortran

[Chris Jones]

> 
> This is what I did supposingly but with xcode11.0... but anyway, I will 
> try again with the new xcode.
> 
> I will maintain you informed... it happened to other people in the 
> office so if I solve this many people will be happy ;o)

Once Xcode is updated to 11.1 please also run

xcode-select --install

to make sure the CLT's are updated as well.

Then, you should check that

xcrun --show-sdk-path

gives a valid 10.15 SDK path.

Chris