Let's try using perl for building proto.h.

mkproto.pl

+# generate prototypes for rsync
+use strict;
+
+my $old_protos = '';
+if (open(IN, '<', 'proto.h')) {
+    $old_protos = join('', <IN>);
+    close IN;
+}
+
+my %FN_MAP = (
+    BOOL => 'BOOL ',
+    CHAR => 'char ',
+    INTEGER => 'int ',
+    STRING => 'char *',
+);
+
+my $inheader = 0;
+my $protos = qq|/* This file is automatically generated with "make proto". DO NOT EDIT */\n\n|;
+
+while (<>) {
+    if ($inheader) {
+	if (/[)][ \t]*$/) {
+	    $inheader = 0;
+	    s/$/;/;
+	}
+	$protos .= $_;
+    }
+
+    if (/^FN_(LOCAL|GLOBAL)_([^(]+)\(([^,()]+)/) {
+	my $ret = $FN_MAP{$2};
+	my $func = $3;
+	my $arg = $1 eq 'LOCAL' ? 'int ' : 'void';
+	$protos .= "$ret$func($arg);\n";
+    } elsif (/^static|^extern/ || /[;]/) {
+	;
+    } elsif (!/^[A-Za-z][A-Za-z0-9_]* /) {
+	;
+    } elsif (/[(].*[)][ \t]*$/) {
+	s/$/;/;
+	$protos .= $_;
+    } elsif (/[(]/) {
+	$inheader = 1;
+	$protos .= $_;
+    }
+}
+
+if ($old_protos ne $protos) {
+    open(OUT, '>', 'proto.h') or die $!;
+    print OUT $protos;
+    close OUT;
+}

prepare-source.mak

@@ -9,12 +9,7 @@ config.h.in: configure.in aclocal.m4
 	autoheader && touch config.h.in
 
 proto.h: *.c lib/compat.c
-	cat *.c lib/compat.c | awk -f mkproto.awk >proto.h.new
-	if diff proto.h proto.h.new >/dev/null 2>&1; then \
-	  rm proto.h.new; \
-	else \
-	  mv proto.h.new proto.h; \
-	fi
+	perl mkproto.pl *.c lib/compat.c
 
 man: rsync.1 rsyncd.conf.5