don't break the build if capability.h isn't present, just disable capabilities

- also, report linux capability support in the configure
  summary if running on linux.
10 jobs for build-nocaps in 8 minutes and 10 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Precheck
passed #21748
amd64 docker linux
precheck:debian:sid:amd64

00:00:10

 
  Build
passed #21749
amd64 docker linux
build:clang:debian:sid:amd64

00:03:00

passed #21751
i386 docker linux
build:clang:debian:sid:i386

00:03:19

passed #21750
amd64 docker linux
build:debian:sid:amd64

00:02:00

passed #21752
i386 docker linux
build:debian:sid:i386

00:02:04

 
  Test
passed #21756
amd64 docker linux
systemtest:debian:sid:amd64

00:04:39

passed #21757
i386 docker linux
systemtest:debian:sid:i386

00:04:39

passed #21754
amd64 docker linux
unittest:clang:debian:sid:amd64

00:00:46

passed #21753
amd64 docker linux
unittest:debian:sid:amd64

00:00:43

passed #21755
i386 docker linux
unittest:debian:sid:i386

00:00:46