build: take compiler options from environment

build: take compiler options from environment

This makes it possible to compile IoTivity with custom compiler
options. This way someone can use optimized compiler settings for the
target CPU. I want to use this in OpenWrt / LEDE to build specific
IoTivity binaries for each CPU architecture, there the build system
provides the matching compiler options.

Change-Id: I86772f73b81c41d7e96e51b434fd9e3b4992753a
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-on: https://gerrit.iotivity.org/gerrit/15435
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Reviewed-by: Mike Fenelon <mike.fenelon@microsoft.com>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>