Log Message:
Fix installation on Darwin with the default PKG_OPTIONS.

Background: On systems with CoreText (i.e., Darwin), pango
unconditionally links with it and installs certain libraries; this
is already handled in PLIST.

Problem: On systems where cairo has been built with the "quartz"
option, pango installs an additional header file.

Fix: Given that the "quartz" option is off by default, and assuming
that users who enable it likely do so globally in PKG_DEFAULT_OPTIONS,
add a nominal "quartz" option to pango that conditionalizes whether
to expect this additional header file.

"Sounds good" from jperkin@.

Tested on:

* Mac OS X 10.9.3, with and without PKG_DEFAULT_OPTIONS+=quartz
  (from cairo on up the dependency chain)
* NetBSD/amd64 6.1.4, with X sets and the default options