extlibs: Removing redundant configuration

extlibs: Removing redundant configuration

Moving global environment configuration common for all
platforms into build_common/SConscript and removing repeated
lines from extlibs/*.

Bug: https://jira.iotivity.org/browse/IOT-1583
Change-Id: Iacd5905d3c221d3d1a142f08a5ea61a899ad08b1
Signed-off-by: Pawel Winogrodzki <pawelwi@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/21299
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
(cherry picked from commit 038ec8e2d700d06722c9db43b41b8c534deb42ce)
Reviewed-on: https://gerrit.iotivity.org/gerrit/21441
Reviewed-by: George Nash <george.nash@intel.com>
Reviewed-by: Rick Bell <richard.s.bell@intel.com>
Reviewed-by: Mats Wichmann <mats@linux.com>