Improved support for Visual Studio

Improved support for Visual Studio

1. Use SCons's MSVC_VERSION variable to support multiple VS
versions, instead of more fragile IoTivity-specific solutions.

2. Don't execute vsvars32.bat "manually" from run.bat. Let SCons
execute that batch script when needed, based on the value of
MSVC_VERSION.

3. Remove VS version handling from gtest/SConscript. Use SCons for
building gtest (and eliminate the need for cmake on Windows).

4. Delete duplicated lines from build_common/windows/SConscript.

Change-Id: I6490a32533624b7959b0a30992c8f10811e7735e
Signed-off-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/11727
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: David Antler <david.a.antler@intel.com>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>