Install Cairo headers if Cairo has been found.

Closes #83.
6 jobs for cairo-headers in 57 minutes and 29 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #235167
build

00:04:17

passed #235172
build_android

00:01:06

passed #235168
build_clang_libcpp

00:04:09

passed #235171
build_clazy

00:57:29

passed #235170
build_mingw64_fedora

00:02:42

passed #235169
build_ubuntu_16_04

00:01:29