Using backslashes and slashes in the strings of the sed 'y' command shall be avoided since its interpretation is not the same across platforms.

Now we use the sed 's' command with a bracket expression.

acinclude.m4

@@ -7,7 +7,7 @@ dnl Check for headers if check not already done.
 AC_DEFUN([CURL_CHECK_HEADERS_ONCE], [
   for f_hdr in $1
   do
-    u_hdr=`echo "$f_hdr" | sed 'y/.\/-/___/'`
+    u_hdr=`echo "$f_hdr" | sed 's/[\-|\.|\/]/_/g'`
     eval prev_check_res=\$ac_cv_header_$u_hdr
     case "$prev_check_res" in
       yes | no)