Fix the build

autogen was failing due to this variable being
defined multiple times.
diff --git a/win32/vs15/Makefile.am b/win32/vs15/Makefile.am
index 94408c6..fcea2b3 100644
--- a/win32/vs15/Makefile.am
+++ b/win32/vs15/Makefile.am
@@ -42,6 +42,6 @@
 MSVC_BASE_VER_LONG = 2010
 MSVC_VER = 15
 MSVC_VER_LONG = 15
-MSVC_TOOLSET = 141
+#MSVC_TOOLSET = 141
 
 include $(top_srcdir)/win32/Makefile-newvs.am