gmp: The macOS 10.14 SDK does not appear to be installed (but it is)

[ryandesign (Ryan Carsten Schmidt)]

While upgrading infrastructure ports on the Mojave buildbot worker, I noticed this warning:

Warning: The macOS 10.14 SDK does not appear to be installed. Ports may not build correctly.
Warning: You can install it as part of the Xcode Command Line Tools package by running `xcode-select --install'.
--->  Fetching distfiles for gmp
--->  Attempting to fetch gmp-6.1.2.tar.bz2 from https://distfiles.macports.org/gmp
--->  Verifying checksums for gmp
--->  Extracting gmp
--->  Applying patches to gmp
--->  Configuring gmp
--->  Building gmp
--->  Staging gmp into destroot
--->  Installing gmp @6.1.2_3
--->  Cleaning gmp
Warning: The macOS 10.14 SDK does not appear to be installed. Ports may not build correctly.
Warning: You can install it as part of the Xcode Command Line Tools package by running `xcode-select --install'.
--->  Deactivating gmp @6.1.2_1
--->  Cleaning gmp
--->  Activating gmp @6.1.2_3
--->  Cleaning gmp

This machine has Xcode 10.3 and the Xcode 10.3 version of the command line tools installed, so the macOS 10.14 SDK does exist in both of the usual places:

$ ls -ld {/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer,/Library/Developer/CommandLineTools}/SDKs/MacOSX10.14.sdk
lrwxr-xr-x  1 root  wheel  10 Sep 20 22:36 /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk -> MacOSX.sdk
lrwxr-xr-x  1 root  wheel  10 Oct 23 18:36 /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk -> MacOSX.sdk

This warning only appeared when upgrading the gmp port, not when upgrading other ports.

I think this problem may be happening because the gmp port clears configure.sdkroot.

[jmroot (Joshua Root)]

Yeah, the check generating the warning just doesn't take into account that the portfile may have overridden the sdkroot.

[ryandesign (Ryan Carsten Schmidt)]

I wasn't sure whether ports overriding sdkroot (or specifically clearing sdkroot) was something base intended to allow.

[MarcusCalhoun-Lopez (Marcus Calhoun-Lopez)]

In 175c44aea6ee49fd4f55ad871b8b131bf23f12c5/macports-ports (master):

gmp: avoid clearing configure.sdkroot

Fixes #59503