Restore cross-platform GNU Make build

README

@@ -88,13 +88,17 @@ 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.
+lib/. Platform-specific subdirectories allow Linux, macOS and Windows builds
+to coexist.
 
 On Linux:
 
     make linux
 
+On macOS:
+
+    make macos
+
 To cross-compile for 64-bit Windows on Linux (requires MinGW-w64):
 
     make windows
@@ -103,9 +107,9 @@ 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.
+The resulting files are in lib/linux/, lib/macos/ 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