Add Linux and MinGW build support

README

@@ -85,6 +85,28 @@ http://people.qualcomm.com/karn
 This software may be used under the terms of the GNU Lesser General
 Public License (LGPL); see the file lesser.txt for details.
 
+BUILDING
+
+The GNU Make build keeps intermediate files under obj/ and libraries under
+lib/. Platform-specific subdirectories allow Linux and Windows builds to
+coexist.
+
+On Linux:
+
+    make linux
+
+To cross-compile for 64-bit Windows on Linux (requires MinGW-w64):
+
+    make windows
+
+To build natively on Windows, open an MSYS2 MinGW 64-bit shell and run:
+
+    make
+
+The resulting files are in lib/linux/ or lib/windows/. Use `make static` or
+`make shared` to build only one library type, and `make clean` to remove all
+generated files.
+
 Revision history:
 Version 1.0 released 29 May 2001