build: Accomodate makensis v2.x

Apparently the -X flag doesn't work as expected in makensis v2.x

For example:

  makensis -V2 share/setup.nsi -X'OutFile "test.exe"'

Will output:

  OutFile expects 1 parameters, got 0.
  Usage: OutFile install_output.exe

So let's instead construct the file using POSIX-compliant commands and
shell constructs

Makefile.am

@@ -80,7 +80,7 @@ $(BITCOIN_WIN_INSTALLER): all-recursive
 	STRIPPROG="$(STRIP)" $(INSTALL_STRIP_PROGRAM) $(BITCOIN_CLI_BIN) $(top_builddir)/release
 	STRIPPROG="$(STRIP)" $(INSTALL_STRIP_PROGRAM) $(BITCOIN_TX_BIN) $(top_builddir)/release
 	STRIPPROG="$(STRIP)" $(INSTALL_STRIP_PROGRAM) $(BITCOIN_WALLET_BIN) $(top_builddir)/release
-	@test -f $(MAKENSIS) && $(MAKENSIS) -V2 $(top_builddir)/share/setup.nsi -X'OutFile "$@"' || \
+	@test -f $(MAKENSIS) && echo 'OutFile "$@"' | cat $(top_builddir)/share/setup.nsi - | $(MAKENSIS) -V2 - || \
 	  echo error: could not build $@
 	@echo built $@