Check for liblzma headers in configure too and add wxUSE_LIBLZMA

Prepare for using liblzma in wxWidgets code by adding the necessary
option and updating the configure check for it (which had been already
present due to libtiff possible dependency on liblzma).

No real changes yet.

include/wx/gtk/setup0.h

@@ -611,6 +611,22 @@
 // wxUSE_LIBPNG
 #define wxUSE_ZLIB          1
 
+// Set to 1 if liblzma is available to enable wxLZMA{Input,Output}Stream
+// classes.
+//
+// Notice that if you enable this build option when not using configure or
+// CMake, you need to ensure that liblzma headers and libraries are available
+// (i.e. by building the library yourself or downloading its binaries) and can
+// be found, either by copying them to one of the locations searched by the
+// compiler/linker by default (e.g. any of the directories in the INCLUDE or
+// LIB environment variables, respectively, when using MSVC) or modify the
+// make- or project files to add references to these directories.
+//
+// Default is 0 under MSW, auto-detected by configure.
+//
+// Recommended setting: 1 if you need LZMA compression.
+#define wxUSE_LIBLZMA       0
+
 // If enabled, the code written by Apple will be used to write, in a portable
 // way, float on the disk. See extended.c for the license which is different
 // from wxWidgets one.