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

lib-src/expat/win32/README.txt

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+
+Expat can be built on Windows in three ways: 
+  using MS Visual C++ (6.0 or .NET), Borland C++ Builder 5 or Cygwin.
+
+* Cygwin:
+  This follows the Unix build procedures.
+
+* C++ Builder 5:
+  Possible with make files in the BCB5 subdirectory.
+  Details can be found in the ReadMe file located there.
+
+* MS Visual C++ 6:
+  Based on the workspace file expat.dsw. The related project
+  files (.dsp) are located in the lib subdirectory.
+
+* MS Visual Studio .NET 2002, 2003, 2005, 2008, 2010:
+  The VC++ 6 workspace file (expat.dsw) and project files (.dsp)
+  can be opened and imported in VS.NET without problems.
+
+* All MS C/C++ compilers:
+  The output for all projects will be generated in the win32\bin
+  directory, intermediate files will be located in project-specific
+  subdirectories of win32\tmp.
+  
+* Creating MinGW dynamic libraries from MS VC++ DLLs:
+  
+  On the command line, execute these steps:
+  pexports libexpat.dll > expat.def
+  pexports libexpatw.dll > expatw.def
+  dlltool -d expat.def -l libexpat.a
+  dlltool -d expatw.def -l libexpatw.a
+  
+  The *.a files are mingw libraries.
+
+* Special note about MS VC++ and runtime libraries:
+
+  There are three possible configurations: using the
+  single threaded or multithreaded run-time library,
+  or using the multi-threaded run-time Dll. That is, 
+  one can build three different Expat libraries depending
+  on the needs of the application.
+
+  Dynamic Linking:
+
+  By default the Expat Dlls are built to link statically
+  with the multi-threaded run-time library. 
+  The libraries are named
+  - libexpat(w).dll 
+  - libexpat(w).lib (import library)
+  The "w" indicates the UTF-16 version of the library.
+
+  One rarely uses other versions of the Dll, but they can
+  be built easily by specifying a different RTL linkage in
+  the IDE on the C/C++ tab under the category Code Generation.
+
+  Static Linking:
+
+  The libraries should be named like this:
+  Single-theaded:     libexpat(w)ML.lib
+  Multi-threaded:     libexpat(w)MT.lib
+  Multi-threaded Dll: libexpat(w)MD.lib
+  The suffixes conform to the compiler switch settings
+  /ML, /MT and /MD for MS VC++.
+  
+  Note: In Visual Studio 2005 (Visual C++ 8.0) and later, the
+  single-threaded runtime library is not supported anymore.
+
+  By default, the expat-static and expatw-static projects are set up
+  to link statically against the multithreaded run-time library,
+  so they will build libexpatMT.lib or libexpatwMT.lib files.
+
+  To build the other versions of the static library, 
+  go to Project - Settings:
+  - specify a different RTL linkage on the C/C++ tab
+    under the category Code Generation.
+  - then, on the Library tab, change the output file name
+    accordingly, as described above
+
+  An application linking to the static libraries must
+  have the global macro XML_STATIC defined.