Update detectenv-msvc.mak (#316)

Windows x86 ARM capability
diff --git a/win32/detectenv-msvc.mak b/win32/detectenv-msvc.mak
index a94ab84..83d8786 100644
--- a/win32/detectenv-msvc.mak
+++ b/win32/detectenv-msvc.mak
@@ -119,6 +119,9 @@
 
 !if "$(PLAT)" == "x64"
 LDFLAGS_ARCH = /machine:x64
+!elseif "$(PLAT)" == "arm"
+LDFLAGS_ARCH = /machine:arm
+CFLAGS_ADD = $(CFLAGS_ADD) /DWINAPI_FAMILY=3
 !else
 LDFLAGS_ARCH = /machine:x86
 !endif