Improve the frustrating gtest SConscript
Improve the frustrating gtest SConscript

Define gtest version in a single variable, for future-proofing
Fix to "python style" (except long lines and constant names) - indents, etc.
Fix redefinition of builtin name 'var' (windows section)
Quit right away if target does not need gtest, and remove extra checks
  for this elsewhere
Add checks for help and clean targets: should not unpack/config/build in
  these cases, and actually clean if clean is called
Split configure and build targets for non-windows:
  may still need to build even if already configured (due to clean target)

Change-Id: Ic9818fdd221eb2d3d7fead8c9585c41db80b3d5a
Signed-off-by: Mats Wichmann <7a5e6f27299d2bbd9d04b59d9e8e4f8eab83d8cb@linux.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18633
Reviewed-by: Phil Coval <ff46fec9739d31cce915aabcc3a7a2c00eee4d6c@osg.samsung.com>
Tested-by: jenkins-iotivity <d95b56ce41a2e1ac4cecdd398defd7414407cc08@iotivity.org>
Reviewed-by: Dan Mihai <ba8e1833495515b49ded9e20cae80a8129155aa9@microsoft.com>
Origin: master
Reviewed-on: https://gerrit.iotivity.org/gerrit/20905
Reviewed-by: Peter Hatina <c048335d94110f7e68fa5d23c0a0228493973b6c@gmail.com>
Reviewed-by: Ziran Sun <3baa2a3e3c6d96dba4b6f1736d53815847db9a07@samsung.com>

iotivity/iotivity,rzr/iotivity,iotivity/iotivity,rzr/iotivity,iotivity/iotivity,rzr/iotivity,iotivity/iotivity,iotivity/iotivity,iotivity/iotivity,rzr/iotivity,iotivity/iotivity,rzr/iotivity,rzr/iotivity,rzr/iotivity,iotivity/iotivity