gtest: Use whereis instead of which

gtest: Use whereis instead of which

whereis command is more popular that which among GNU/Linux Distros:
Archlinux is not having which by default

For reference debian has them in different packages:

  util-linux: /usr/bin/whereis
  debianutils: /usr/bin/which

Bug: https://jira.iotivity.org/browse/IOT-2084
Change-Id: I1966a31df1281e7f28fb8ea40f3dc4514ba22713
Signed-off-by: Philippe Coval <philippe.coval@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/19345
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>