BinaryProvider can install a package with Julia 0.7 but not with 0.6


I’m trying to switch a package to BinaryBuilder.jl. It works fine on most systems (at least, those that I can test with Travis), but I’m facing some difficulties with the Windows build: the library is successfully installed in Julia 0.7, but fails with Julia 0.6 with the following error:

LoadError: BinaryProvider.LibraryProduct(nothing, String["libwcs"], :libwcs, "Prefix(C:\\Users\\appveyor\\.julia\\v0.6\\WCS\\deps\\usr)") is not satisfied, cannot generate deps.jl!
while loading C:\Users\appveyor\.julia\v0.6\WCS\deps\build.jl, in expression starting on line 46

Here are some information:

Has anyone a clue about why Julia 0.6 fails to install the library on Windows? Am I doing something wrong, maybe in the build script?