hadrian: Don't rely on realpath in bindist Makefile

As noted in #19963, `realpath` is not specified by POSIX and therefore
cannot be assumed to be available. Here we provide a POSIX shell
implementation of `realpath`, due to Julian Ospald and others.

Closes #19963.

hadrian/bindist/Makefile

@@ -202,7 +202,7 @@ update_package_db: install_bin install_lib
 	@echo "$(PKG_CONFS)"
 	@echo "Updating the package DB"
 	$(foreach p, $(PKG_CONFS),\
-		$(call patchpackageconf,$(shell echo $(notdir $p) | sed 's/-\([0-9]*[0-9]\.\)*conf//g'),$(shell echo "$p" | sed 's:xxx:   :g'),$(docdir),$(shell realpath --relative-to="$(ActualLibsDir)" "$(docdir)")))
+		$(call patchpackageconf,$(shell echo $(notdir $p) | sed 's/-\([0-9]*[0-9]\.\)*conf//g'),$(shell echo "$p" | sed 's:xxx:   :g'),$(docdir),$(shell mk/relpath.sh "$(ActualLibsDir)" "$(docdir)")))
 	'$(WrapperBinsDir)/$(CrossCompilePrefix)ghc-pkg' recache
 
 install_mingw:

hadrian/src/Rules/BinaryDist.hs

@@ -337,9 +337,10 @@ compressorExtension Bzip2 = "bz2"
 -- @./configure [...] && make install@ workflow.
 bindistInstallFiles :: [FilePath]
 bindistInstallFiles =
-    [ "config.sub", "config.guess", "install-sh", "mk" -/- "config.mk.in"
-    , "mk" -/- "install.mk.in", "mk" -/- "project.mk", "README"
-    , "INSTALL" ]
+    [ "config.sub", "config.guess", "install-sh"
+    , "mk" -/- "config.mk.in", "mk" -/- "install.mk.in", "mk" -/- "project.mk"
+    , "mk" -/- "relpath.sh"
+    , "README", "INSTALL" ]
 
 -- | This auxiliary function gives us a top-level 'Filepath' that we can 'need'
 -- for all libraries and programs that are needed for a complete build.

mk/relpath.sh

+#!/bin/sh
+
+# POSIX shell implementation of `realpath --relative-to=$1 $2.
+# This is an adaptation of the implementation from
+# <https://github.com/Offirmo/offirmo-shell-lib>.
+
+# returns relative path to $2=$target from $1=$source
+## NOTE : path are compared in text only. They don’t have to exist
+##        and they WONT be normalized/escaped
+## Result in "$return_value"# both $1 and $2 are absolute paths beginning with /
+
+src="$1"
+target="$2"
+
+common_part="$src"
+result=""
+
+while test "${target#$common_part}" = "${target}" ; do
+    #echo "common_part is now : \"$common_part\""
+    #echo "result is now      : \"$result\""
+    #echo "target#common_part : \"${target#$common_part}\""
+    # no match, means that candidate common part is not correct
+    # go up one level (reduce common part)
+    common_part="$(dirname "$common_part")"
+    # and record that we went back
+    if test -z "$result" ; then
+        result=".."
+    else
+        result="../$result"
+    fi
+done
+
+#echo "common_part is     : \"$common_part\""
+
+if test "$common_part" = "/" ; then
+    # special case for root (no common path)
+    result="$result/"
+fi
+
+# since we now have identified the common part,
+# compute the non-common part
+forward_part="${target#$common_part}"
+#echo "forward_part = \"$forward_part\""
+
+if test -n "$result" && test -n "$forward_part" ; then
+    #echo "(simple concat)"
+    result="$result$forward_part"
+elif test -n "$forward_part" ; then
+    #echo "(concat with slash removal)"
+    result="$(printf "%s" "$forward_part" | cut -c 1-)"
+fi
+
+printf "%s" "$result"