Add expat 2.1.0 for Windows, Mac, and for local Linux builds (use system expat otherwise)

lib-src/expat/tests/benchmark/README.txt

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+Use this benchmark command line utility as follows:
+
+  benchmark [-n] <file name> <buffer size> <# iterations>
+
+The command line arguments are:
+
+  -n             ... optional; if supplied, namespace processing is turned on
+  <file name>    ... name/path of test xml file
+  <buffer size>  ... size of processing buffer;
+                     the file is parsed in chunks of this size
+  <# iterations> ... how often will the file be parsed
+
+Returns:
+
+  The time (in seconds) it takes to parse the test file,
+  averaged over the number of iterations.