@value fails with @image for pdf output

[Christopher Dimech]

Please note that the following code fails for pdf output, but works
for html if I put dirhtml at the correct relative path to index.html

@set dir-pdf ../Ideogr/Ch03b--Chmed
@image{@value{dir-pdf}/igc03--01--Sicl--CircAmRg,,,,.png}

@set dir-html ../../Ideogr/Ch03b--Chmed
@image{@value{dir-html}/igc03--01--Sicl--CircAmRg,,,,png}