Install Cairo headers if Cairo has been found.

Closes #83.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8e27bc2..4eaea26 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -662,6 +662,13 @@
       splash/SplashXPathScanner.h
       DESTINATION include/poppler/splash)
   endif()
+  if(CAIRO_FOUND)
+    install(FILES
+      poppler/CairoFontEngine.h
+      poppler/CairoOutputDev.h
+      poppler/CairoRescaleBox.h
+      DESTINATION include/poppler)
+  endif()
 endif()