icu: fix include path returned by icu-config

pkgs/development/libraries/icu/base.nix

@@ -34,6 +34,9 @@ stdenv.mkDerivation {
 
   preConfigure = ''
     sed -i -e "s|/bin/sh|${stdenv.shell}|" configure
+
+    # $(includedir) is different from $(prefix)/include due to multiple outputs
+    sed -i -e 's|^\(CPPFLAGS = .*\) -I\$(prefix)/include|\1 -I$(includedir)|' config/Makefile.inc.in
   '' + stdenv.lib.optionalString stdenv.isArm ''
     # From https://archlinuxarm.org/packages/armv7h/icu/files/icudata-stdlibs.patch
     sed -e 's/LDFLAGSICUDT=-nodefaultlibs -nostdlib/LDFLAGSICUDT=/' -i config/mh-linux