Define PY3_DLLNAME to fix build

PY3_DLLNAME is used in Python/pathconfig.c but isn't
defined. I guess it is useless but it will fix the build.

Co-authored-by: jeremyd2019 <github@jdrake.com>

Author: naveen521kk

configure.ac

@@ -4799,7 +4799,7 @@ then
 	*-*-mingw*)
 	DYNLOADFILE="dynload_win.o"
 	extra_machdep_objs="$extra_machdep_objs PC/dl_nt.o"
-	CFLAGS_NODIST="$CFLAGS_NODIST -DMS_DLL_ID='\"$VERSION\"'"
+	CFLAGS_NODIST="$CFLAGS_NODIST -DMS_DLL_ID='\"$VERSION\"' -DPY3_DLLNAME='L\"$DLLLIBRARY\"'"
 	;;
 	esac
 fi