liblzma: Refer to MinGW-w64 instead of MinGW in the API headers.

MinGW (formely a MinGW.org Project, later the MinGW.OSDN Project
at <https://osdn.net/projects/mingw/>) has GCC 9.2.0 as the
most recent GCC package (released 2021-02-02). The project might
still be alive but majority of people have switched to MinGW-w64.
Thus it seems clearer to refer to MinGW-w64 in our API headers too.
Building with MinGW is likely to still work but I haven't tested it
in the recent years.

src/liblzma/api/lzma.h

@@ -182,11 +182,11 @@
  * against static liblzma on them, don't worry about LZMA_API_STATIC. That
  * is, most developers will never need to use LZMA_API_STATIC.
  *
- * The GCC variants are a special case on Windows (Cygwin and MinGW).
+ * The GCC variants are a special case on Windows (Cygwin and MinGW-w64).
  * We rely on GCC doing the right thing with its auto-import feature,
  * and thus don't use __declspec(dllimport). This way developers don't
  * need to worry about LZMA_API_STATIC. Also the calling convention is
- * omitted on Cygwin but not on MinGW.
+ * omitted on Cygwin but not on MinGW-w64.
  */
 #ifndef LZMA_API_IMPORT
 #	if !defined(LZMA_API_STATIC) && defined(_WIN32) && !defined(__GNUC__)

src/liblzma/api/lzma/version.h

@@ -106,7 +106,7 @@
 		LZMA_VERSION_COMMIT)
 
 
-/* #ifndef is needed for use with windres (MinGW or Cygwin). */
+/* #ifndef is needed for use with windres (MinGW-w64 or Cygwin). */
 #ifndef LZMA_H_INTERNAL_RC
 
 /**