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diff --git a/m4/build-to-host.m4 b/m4/build-to-host.m4
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+# build-to-host.m4 serial 1
+dnl Copyright (C) 2023 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+dnl Written by Bruno Haible.
+
+dnl When the build environment ($build_os) is different from the target runtime
+dnl environment ($host_os), file names may need to be converted from the build
+dnl environment syntax to the target runtime environment syntax. This is
+dnl because the Makefiles are executed (mostly) by build environment tools and
+dnl therefore expect file names in build environment syntax, whereas the runtime
+dnl expects file names in target runtime environment syntax.
+dnl
+dnl For example, if $build_os = cygwin and $host_os = mingw32, filenames need
+dnl be converted from Cygwin syntax to native Windows syntax:
+dnl   /cygdrive/c/foo/bar -> C:\foo\bar
+dnl   /usr/local/share    -> C:\cygwin64\usr\local\share
+dnl
+dnl gl_BUILD_TO_HOST([somedir])
+dnl This macro takes as input an AC_SUBSTed variable 'somedir', which must
+dnl already have its final value assigned, and produces two additional
+dnl AC_SUBSTed variables 'somedir_c' and 'somedir_c_make', that designate the
+dnl same file name value, just in different syntax:
+dnl   - somedir_c       is the file name in target runtime environment syntax,
+dnl                     as a C string (starting and ending with a double-quote,
+dnl                     and with escaped backslashes and double-quotes in
+dnl                     between).
+dnl   - somedir_c_make  is the same thing, escaped for use in a Makefile.
+
+AC_DEFUN([gl_BUILD_TO_HOST],
+[
+  AC_REQUIRE([AC_CANONICAL_BUILD])
+  AC_REQUIRE([AC_CANONICAL_HOST])
+  AC_REQUIRE([gl_BUILD_TO_HOST_INIT])
+
+  dnl Define somedir_c.
+  gl_final_[$1]="$[$1]"
+  dnl Translate it from build syntax to host syntax.
+  case "$build_os" in
+    cygwin*)
+      case "$host_os" in
+        mingw*)
+          gl_final_[$1]=`cygpath -w "$gl_final_[$1]"` ;;
+      esac
+      ;;
+  esac
+  dnl Convert it to C string syntax.
+  [$1]_c=`echo "$gl_final_[$1]" | sed -e "$gl_sed_double_backslashes" -e "$gl_sed_escape_doublequotes"`
+  [$1]_c='"'"$[$1]_c"'"'
+  AC_SUBST([$1_c])
+
+  dnl Define somedir_c_make.
+  [$1]_c_make=`echo "$[$1]_c" | sed -e "$gl_sed_escape_for_make_1" -e "$gl_sed_escape_for_make_2"`
+  dnl Use the substituted somedir variable, when possible, so that the user
+  dnl may adjust somedir a posteriori when there are no special characters.
+  if test "$[$1]_c_make" = '\"'"${gl_final_[$1]}"'\"'; then
+    [$1]_c_make='\"$([$1])\"'
+  fi
+  AC_SUBST([$1_c_make])
+])
+
+dnl Some initializations for gl_BUILD_TO_HOST.
+AC_DEFUN([gl_BUILD_TO_HOST_INIT],
+[
+  gl_sed_double_backslashes='s/\\/\\\\/g'
+  gl_sed_escape_doublequotes='s/"/\\"/g'
+changequote(,)dnl
+  gl_sed_escape_for_make_1="s,\\([ \"&'();<>\\\\\`|]\\),\\\\\\1,g"
+changequote([,])dnl
+  gl_sed_escape_for_make_2='s,\$,\\$$,g'
+])