Document VSINSTALLDIR path WAR

buildscripts/incremental/build.cmd

@@ -1,6 +1,20 @@
 
 call activate %CONDA_ENV%
 
+@rem LLVM derives the location of diaguids.lib from the build-time environment.
+@rem Conda-forge packaging works around this by substituting the build-time
+@rem location of Visual Studio with $ENV{VSINSTALLDIR}. In order to ensure that
+@rem this environment variable is set appropriately, we activate the Visual
+@rem Studio Developer Command Prompt prior to running setup.py
+@rem
+@rem This workaround is required whilst using LLVM from conda-forge; it may also
+@rem be necessary to consider a workaround for our own llvmdev packages.
+@rem
+@rem For more info, see:
+@rem
+@rem - https://github.com/conda-forge/llvmdev-feedstock/issues/175
+@rem - https://github.com/conda-forge/llvmdev-feedstock/pull/223
+@rem - https://github.com/MicrosoftDocs/visualstudio-docs/issues/7774
 if "%LLVM%"=="15" (
   call "C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\Common7\Tools\VsDevCmd.bat"
   if %errorlevel% neq 0 exit /b %errorlevel%