gnu: xmonad: Add patch to support dynamic linking.

Fixes <https://bugs.gnu.org/43131>.

* gnu/packages/patches/xmonad-dynamic-linking.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/wm.scm (xmonad)[source]: Use it.

gnu/local.mk

@@ -1676,6 +1676,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/xf86-video-voodoo-pcitag.patch		\
   %D%/packages/patches/xfce4-panel-plugins.patch		\
   %D%/packages/patches/xfce4-settings-defaults.patch		\
+  %D%/packages/patches/xmonad-dynamic-linking.patch		\
   %D%/packages/patches/xplanet-1.3.1-cxx11-eof.patch		\
   %D%/packages/patches/xplanet-1.3.1-libdisplay_DisplayOutput.cpp.patch	\
   %D%/packages/patches/xplanet-1.3.1-libimage_gif.c.patch	\

gnu/packages/patches/xmonad-dynamic-linking.patch

@@ -0,0 +1,16 @@
+This patch is required for xmonad to make use of shared libraries.
+Without it, xmonad will not work since we do not (by default) use
+statically linked Haskell libraries.
+
+diff -ruN xmonad-0.15-a/src/XMonad/Core.hs xmonad-0.15-b/src/XMonad/Core.hs
+--- xmonad-0.15-a/src/XMonad/Core.hs	1969-12-31 19:00:00.000000000 -0500
++++ xmonad-0.15-b/src/XMonad/Core.hs	1969-12-31 19:00:00.000000000 -0500
+@@ -681,6 +681,8 @@
+        compileGHC bin dir errHandle =
+          runProcess "ghc" ["--make"
+                           , "xmonad.hs"
++                          , "-dynamic"
++                          , "-fPIC"
+                           , "-i"
+                           , "-ilib"
+                           , "-fforce-recomp"

gnu/packages/wm.scm

@@ -655,7 +655,8 @@ (define-public xmonad
                                   "xmonad-" version ".tar.gz"))
               (sha256
                (base32
-                "0a7rh21k9y6g8fwkggxdxjns2grvvsd5hi2ls4klmqz5xvk4hyaa"))))
+                "0a7rh21k9y6g8fwkggxdxjns2grvvsd5hi2ls4klmqz5xvk4hyaa"))
+              (patches (search-patches "xmonad-dynamic-linking.patch"))))
     (build-system haskell-build-system)
     (inputs
      `(("ghc-extensible-exceptions" ,ghc-extensible-exceptions)