Cleaned up cmake_vs20xx.bat.

cmake_vs2008.bat

@@ -1,4 +1,3 @@
 @del /F CMakeCache.txt
-@set "arch="
-@if "%1" == "Win64" set "arch= %1"
+@if "%1" == "Win64" (set "arch= %1") else (set "arch=")
 cmake -G "Visual Studio 9 2008%arch%"

cmake_vs2010.bat

@@ -1,4 +1,3 @@
 @del /F CMakeCache.txt
-@set "arch="
-@if "%1" == "Win64" set "arch= %1"
+@if "%1" == "Win64" (set "arch= %1") else (set "arch=")
 cmake -G "Visual Studio 10%arch%"

cmake_vs2012.bat

@@ -1,4 +1,3 @@
 @del /F CMakeCache.txt
-@set "arch="
-@if "%1" == "Win64" set "arch= %1"
+@if "%1" == "Win64" (set "arch= %1") else (set "arch=")
 cmake -G "Visual Studio 11%arch%"