Merge/Dependencies.hs: added 'icu' to extralibs

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Merge/Dependencies.hs: added 'imagemagick' to pkgconfig libs

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Merge.hs: clarify error message when hackport fails to find suitable ghc version

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>