Install Cairo headers if Cairo has been found.

Closes #83.
6 jobs for cairo-headers in 3 minutes and 32 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #374995
build

00:03:28

passed #375000
build_android

00:01:03

passed #374996
build_clang_libcpp

00:03:21

passed #374999
build_clazy

00:03:18

passed #374998
build_mingw64_fedora

00:03:32

passed #374997
build_ubuntu_16_04

00:01:58