Install directory option ignored on Windows

[rasa]

Both
HaskellPlatform-8.4.3-core-x86_64-setup.exe /D=C:\haskell
and
HaskellPlatform-8.4.3-core-x86_64-setup.exe /S /D=C:\haskell
install in C:\Program Files.
If

haskell-platform/hptool/os-extras/win/templates/Extralibs.nsi.mu

Line 90 in 5b4d97d

${IfNot} ${Silent}

was replaced with IfSilent it might fix the issue.

[randen]

For HP 8.4.3, we had to change the installer a bit for #279 which unfortunately broke the /S functionality (the changes are in PR #308). With NSIS, it is non-trivial to distinguish whether the user specified /D= or not (since the NSIS framework strips it out of the params it makes available to the client), so we never supported just the /D= functionality, but /D= used with /S is supported and will be fixed.