Explain how to circumvent the odd pkg-config installation on MacOSX

See also the conf-cairo package.

Author: Chris00

.travis.yml

@@ -6,6 +6,7 @@ env:
   global:
     - PACKAGE=cairo2
     - DEPOPTS='conf-freetype'
+    - PKG_CONFIG_PATH=/usr/local/opt/libffi/lib/pkgconfig
   matrix:
     - OCAML_VERSION=4.02
     - OCAML_VERSION=4.03

cairo2.opam

@@ -27,6 +27,9 @@ conflicts: [
   "cairo" {= "0.4.1"}
   "cairo" {= "0.4.2"}
 ]
+post-messages: [
+  "Try to re-run the install command with PKG_CONFIG_PATH pointing a pkg-config path including libffi, e.g. if you use homebrew you can try PKG_CONFIG_PATH=/usr/local/opt/libffi/lib/pkgconfig" {failure & os = "macos"}
+]
 synopsis: "Binding to Cairo, a 2D Vector Graphics Library"
 description: """
 This is a binding to Cairo, a 2D graphics library with support for