build-support/cabal: add an option to enable shared library support

enableSharedLibraries configures Cabal to build of shared libraries. This
option requires that all dependencies of the package have been compiled
for use in shared libraries, too.

enableSharedExecutables configures Cabal to prefer shared libraries when
linking executables.

This patch partly fixes issue #1084.

pkgs/build-support/cabal/default.nix

···
 
 { stdenv, fetchurl, lib, pkgconfig, ghc, Cabal, jailbreakCabal, glibcLocales
 , enableLibraryProfiling ? false
+, enableSharedLibraries ? false
+, enableSharedExecutables ? false
 , enableCheckPhase ? true
 }:
 
···
             # if that is not desired (for applications), name can be set to
             # fname.
             name = if self.isLibrary then
-                     if enableLibraryProfiling then
+                     if enableLibraryProfiling && self.enableSharedLibraries then
+                       "haskell-${self.pname}-ghc${ghc.ghc.version}-${self.version}-profiling-shared"
+                     else if enableLibraryProfiling && !self.enableSharedLibraries then
                        "haskell-${self.pname}-ghc${ghc.ghc.version}-${self.version}-profiling"
+                     else if !enableLibraryProfiling && self.enableSharedLibraries then
+                       "haskell-${self.pname}-ghc${ghc.ghc.version}-${self.version}-shared"
                      else
                        "haskell-${self.pname}-ghc${ghc.ghc.version}-${self.version}"
                    else
···
             # and run any regression test suites the package might have
             doCheck = enableCheckPhase;
 
+            # pass the '--enable-shared' flag to cabal in the configure
+            # stage to enable building shared libraries
+            inherit enableSharedLibraries;
+
+            # pass the '--enable-executable-dynamic' flag to cabal in
+            # the configure stage to enable linking shared libraries
+            inherit enableSharedExecutables;
+
             extraConfigureFlags = [
               (stdenv.lib.enableFeature enableLibraryProfiling "library-profiling")
+              (stdenv.lib.enableFeature self.enableSharedLibraries "shared")
+              (stdenv.lib.enableFeature self.enableSharedExecutables "executable-dynamic")
               (stdenv.lib.enableFeature self.enableSplitObjs "split-objs")
             ] ++ stdenv.lib.optional (stdenv.lib.versionOlder "7" ghc.ghcVersion) (stdenv.lib.enableFeature self.doCheck "tests");