There's a new field for .cabal files: 

     install-includes: foo.h bar.h

This means the same as 'includes', except that the files named therein
will be installed into $libdir/include.  'includes' should only be
used for headers already installed on the system.

Directories listed in 'include-dirs' still turn into -I options for
hsc2hs, cpphs, and C compilations.  However, for installation
purposes, relative directories in 'include-dirs' are now treated
differently from absolute directories:

  - an absolute directory is copied to the include-dirs field
    of the installed package config

  - files names in install-includes are assumed to be found in
    one of the *relative* directories listed in include-dirs

So the common pattern for providing a header file that you want to
be available everywhere including to via-C compilations against this
package:

  include-dirs: myincludes
  install-includes: foo.h

will install the header file myincludes/foo.h in
$libdir/include/foo.h.