build: Add conditional compilation

Two conditionals are introduced in this commit:
- if <ifaddrs.h> is not available, NetworkInterface helper will always return
  an empty set of interfaces
- If dropping/elevating effective user/group is not supported, an error
  will be thrown if used (e.g., if general.user or general.group is
  configured)

Both conditionals are necessary on Android platform.

Change-Id: Ib360e03514af97ed2d68032fbcbe279a8dc84682
diff --git a/.jenkins.d/20-tests.sh b/.jenkins.d/20-tests.sh
index 6385fc9..a02a94c 100755
--- a/.jenkins.d/20-tests.sh
+++ b/.jenkins.d/20-tests.sh
@@ -25,6 +25,7 @@
 # Run unit tests
 # Core
 ./build/unit-tests-core -l test_suite
+sudo ./build/unit-tests-core -t CorePrivilegeHelper -l test_suite
 
 # Daemon
 ./build/unit-tests-daemon -l test_suite