base uses incorrect SDK for Big Sur 11.1 and Xcode 12.3

[mf2k (Frank Schima)]

base needs to be update to correctly use the 11.1 SDK on Big Sure 11.1 and Xcode 12.3.

On many ports I see configure logs with this:

:debug:configure LDFLAGS='-L/opt/local/lib -Wl,-headerpad_max_install_names -Wl,-syslibroot,/Library/Developer/CommandLineTools/SDKs/MacOSX11.0.sdk -arch x86_64'

But that is not correct. It should be 11.1.

$ ls -l /Library/Developer/CommandLineTools/SDKs
total 0
lrwxr-xr-x  1 root  wheel   14 Dec 18 08:22 MacOSX.sdk -> MacOSX11.1.sdk
drwxr-xr-x  7 root  wheel  224 Nov 12  2019 MacOSX10.14.sdk
drwxr-xr-x  8 root  wheel  256 Jul  9 10:33 MacOSX10.15.sdk
drwxr-xr-x  4 root  wheel  128 Dec 14 13:41 MacOSX11.0.sdk
drwxr-xr-x  7 root  wheel  224 Nov 30 05:33 MacOSX11.1.sdk

I hacked my local copy of portconfigure.tcl so that SDKROOT is 11.1 and many ports build again. But I'm unsure of the proper fix to include the minor version if macOS for the SDK.