* Use IPv6 header qcommon/wspiapi.h if it has been added by the end user

* Update README to explain the Windows XP requirement

Makefile

@@ -423,14 +423,10 @@ ifeq ($(PLATFORM),mingw32)
   BASE_CFLAGS = -Wall -fno-strict-aliasing -Wimplicit -Wstrict-prototypes \
     -DUSE_ICON
 
-  # Require Windows XP or later
-  #
-  # IPv6 support requires a header wspiapi.h to work on earlier versions of
-  # windows. There is no MinGW equivalent of this header so we're forced to
-  # require XP. In theory this restriction can be removed if this header is
-  # obtained separately from the platform SDK. The MSVC build does not have
-  # this limitation.
+  # In the absence of wspiapi.h, require Windows XP or later
+  ifeq ($(shell test -e $(CMDIR)/wspiapi.h; echo $$?),1)
     BASE_CFLAGS += -DWINVER=0x501
+  endif
 
   ifeq ($(USE_OPENAL),1)
     BASE_CFLAGS += -DUSE_OPENAL

README

@@ -433,6 +433,14 @@ PNG support
 
   Restart GtkRadiant and PNG textures are now available.
 
+Building with MinGW for pre Windows XP
+  IPv6 support requires a header named "wspiapi.h" to abstract away from
+  differences in earlier versions of Windows' IPv6 stack. There is no MinGW
+  equivalent of this header and the Microsoft version is obviously not
+  redistributable, so in its absence we're forced to require Windows XP.
+  However if this header is acquired separately and placed in the qcommon/
+  directory, this restriction is lifted.
+
 
 ------------------------------------------------------------- Contributing -----
 

code/qcommon/net_ip.c

@@ -27,7 +27,14 @@ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 #	include <winsock2.h>
 #	include <ws2tcpip.h>
 #	if WINVER < 0x501
+#		ifdef __MINGW32__
+			// wspiapi.h isn't available on MinGW, so if it's
+			// present it's because the end user has added it
+			// and we should look for it in our tree
+#			include "wspiapi.h"
+#		else
 #			include <wspiapi.h>
+#		endif
 #	else
 #		include <ws2spi.h>
 #	endif