add propmerge utility, which merges <PropertyList> XML files. Writes

result to the given output file if specified, or prints it to stdout
otherwise. Usage:

  $ propmerge [-o <outfile>] <list-of-infiles>

utils/propmerge/Makefile.am

@@ -0,0 +1,4 @@
+noinst_PROGRAMS = propmerge
+
+propmerge_SOURCES = propmerge.cxx
+propmerge_LDADD = -lsgprops -lsgxml -lsgio -lsgmisc -lsgdebug -lsgstructure

utils/propmerge/propmerge.cxx

@@ -0,0 +1,70 @@
+#include <iostream>
+#include <string>
+
+#include <simgear/props/props.hxx>
+#include <simgear/props/props_io.hxx>
+#include <simgear/structure/exception.hxx>
+
+using namespace std;
+
+
+void usage()
+{
+	cerr << "Usage:  propmerge [-o <outfile>] <infiles>" << endl;
+}
+
+
+int main(int argc, char *argv[])
+{
+	if (argc < 2) {
+		usage();
+		return -1;
+	}
+
+	int numfiles = 0;
+	string out;
+	SGPropertyNode root;
+
+	for (int i = 1; i < argc; i++) {
+		string s = argv[i];
+		if (s == "-h" || s == "--help") {
+			usage();
+			return 0;
+		}
+
+		if (s == "-o" || s == "--output") {
+			if (i + 1 == argc)
+				break;
+			out = argv[++i];
+			continue;
+		}
+
+		try {
+			readProperties(s, &root);
+			numfiles++;
+		} catch (const sg_exception &e) {
+			cerr << "Error: " << e.getFormattedMessage() << endl;
+			return -2;
+		}
+	}
+
+	if (!numfiles) {
+		cerr << "Error: Nothing to merge." << endl;
+		return -3;
+	}
+
+	try {
+		if (out.empty())
+			writeProperties(cout, &root, true);
+		else
+			writeProperties(out, &root, true);
+
+	} catch (const sg_exception &e) {
+		cerr << "Error: " << e.getFormattedMessage() << endl;
+		return -4;
+	}
+
+	return 0;
+}
+
+