BUILD: use thememan.in instead of Makefile echo

configure.ac

@@ -969,6 +969,7 @@ src/common/dbus/Makefile
 src/fe-text/Makefile
 src/fe-gtk/Makefile
 src/htm/Makefile
+src/htm/thememan
 src/pixmaps/Makefile
 plugins/Makefile
 plugins/python/Makefile

src/htm/Makefile.am

@@ -6,9 +6,7 @@ themedir = $(bindir)
 thememan.exe: htm-mono.csproj
 	$(MDTOOL) $(MDTOOL_OPTS) build $<
 
-thememan:
-	@echo '#!/bin/sh' > $@
-	@echo 'exec mono "$(bindir)/thememan.exe" "$$@"' >> $@
-
 clean-local:
 	rm -f thememan.exe thememan.exe.config thememan.exe.mdb thememan Main.resources
+
+EXTRA_DIST = thememan.in

src/htm/thememan.in

@@ -0,0 +1,3 @@
+#!/bin/sh
+exec_prefix="@exec_prefix@"
+exec mono "@bindir@/thememan.exe" "$@"