Add override variable for CMake library type

This adds the GLFW_LIBRARY_TYPE CMake cache variable, which allows users
and higher-level projects to set what type of library GLFW is built as.

When not empty, this value overrides the standard BUILD_SHARED_LIBS
option for GLFW while still allowing it to control the type of other
libraries in a larger project.

This also allows building GLFW as an object library without adding dummy
source files (as required by Xcode) or producing unused library
binaries.

Projects using CMake 3.12 or later can link the resulting GLFW object
library normally using target_link_libraries.

Fixes #279.
Related to #1307.
Closes #1497.
Closes #1574.
Closes #1928.

README.md

@@ -129,6 +129,8 @@ information on what to include when reporting a bug.
    values to select ANGLE backend (#1380)
  - Added `GLFW_X11_XCB_VULKAN_SURFACE` init hint for selecting X11 Vulkan
    surface extension (#1793)
+ - Added `GLFW_LIBRARY_TYPE` CMake variable for overriding the library type
+   (#279,#1307,#1497,#1574,#1928)
  - 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