Export pkg-config dependencies via the CMake cache

This will be useful when building GLFW as a CMake object library and
linking it into a larger library that exports pkg-config information.

Partly based on #1307 by @a3f.

Closes #1307.

README.md

@@ -131,6 +131,8 @@ information on what to include when reporting a bug.
    surface extension (#1793)
  - Added `GLFW_LIBRARY_TYPE` CMake variable for overriding the library type
    (#279,#1307,#1497,#1574,#1928)
+ - Added `GLFW_PKG_CONFIG_REQUIRES_PRIVATE` and `GLFW_PKG_CONFIG_LIBS_PRIVATE` CMake
+   variables exposing pkg-config dependencies (#1307)
  - Made joystick subsystem initialize at first use (#1284,#1646)
  - Made `GLFW_DOUBLEBUFFER` a read-only window attribute
  - Updated the minimum required CMake version to 3.1
@@ -322,6 +324,7 @@ skills.
  - Fredrik Ehnbom
  - Robin Eklind
  - Siavash Eliasi
+ - Ahmad Fatoum
  - Felipe Ferreira
  - Michael Fogleman
  - Gerald Franz