Fix framework paths on OS X
Fix framework paths on OS X

I audited for uses of `IsUnix` and found this one, which should apply
equally to OS X systems as Linux ones.

chipitsine/msbuild,rainersigwald/msbuild,mono/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,Microsoft/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,mono/msbuild,sean-gilliam/msbuild,chipitsine/msbuild,rainersigwald/msbuild,jeffkl/msbuild,jeffkl/msbuild,rainersigwald/msbuild,mono/msbuild,mono/msbuild,sean-gilliam/msbuild,Microsoft/msbuild,AndyGerlicher/msbuild,chipitsine/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,Microsoft/msbuild,Microsoft/msbuild,sean-gilliam/msbuild,chipitsine/msbuild,cdmihai/msbuild,jeffkl/msbuild