documentation fails to build on Appveyor

I'm splitting this out from the original #2145 (closed). Xref #3133 (closed).

All the required packages are installed on Appveyor but ghostscript fails to run:

[489/1205] Generating genimages/client.png
FAILED: api/docs/genimages/client.png 
cmd.exe /C "cd /D C:\projects\dynamorio\build\build_debug-internal-32\api\docs\genimages && C:\projects\install\WinFIG\fig2dev.exe -L tiff -m 4 C:/projects/dynamorio/api/docs/client.fig client.tiff && C:\ProgramData\chocolatey\bin\mogrify.exe -resize 33% -format png C:/projects/dynamorio/build/build_debug-internal-32/api/docs/genimages/client.tiff[0] && "C:\Program Files (x86)\CMake\bin\cmake.exe" -E remove C:/projects/dynamorio/build/build_debug-internal-32/api/docs/genimages/client.tiff"
'-q' is not recognized as an internal or external command,
operable program or batch file.
fig2dev: changed dir to .
Error in ghostcript command
command was:  -q -dSAFER -sDEVICE=tiff24nc -r80 -g806x846 -sOutputFile="C:\projects\dynamorio\build\build_debug-internal-32\api\docs\genimages\client.tiff"  -