I suggest the following change, to avoid unnecessary temporary files at build time.

Index: Makefile.in
==================================================================
--- Makefile.in
+++ Makefile.in
@@ -36,14 +36,12 @@

 # Create a C-source-ified version of the script resources
 # for TclTLS so that we only need a single file to enable
 # this extension
 tls.tcl.h: @srcdir@/tls.tcl Makefile
-       od -A n -v -t xC < '@srcdir@/tls.tcl' > tls.tcl.h.new.1
-       sed 's@  *@@g;s@..@0x&, @g' < tls.tcl.h.new.1 > tls.tcl.h.new.2
-       rm -f tls.tcl.h.new.1
-       mv tls.tcl.h.new.2 tls.tcl.h
+       od -A n -v -t xC < '@srcdir@/tls.tcl' | \
+       sed 's@  *@@g;s@..@0x&, @g' > tls.tcl.h

 # Create default DH parameters
 dh_params.h: @srcdir@/gen_dh_params Makefile
        @srcdir@/gen_dh_params @GEN_DH_PARAMS_ARGS@ > dh_params.h.new
        mv dh_params.h.new dh_params.h

The reason it's done this way is because the exit status is only of the last command in the pipeline. Your solution will only fail if 'sed' fails, not if 'od' fails. The result will appear to be successful but be a zero byte file.