Also include some missing CMake utilities for install logic.

… defs.

Not sure how I missed this.  Prior to this commit the install logic
will succeed, but the previous target_include_directories calls
with $<INSTALL_INTERFACE> evaluate variables not yet defined.

Previously:

```cmake
set_target_properties(nanogui PROPERTIES
  INTERFACE_COMPILE_DEFINITIONS
"NANOGUI_USE_OPENGL;NANOGUI_SHARED;NVG_SHARED"
  INTERFACE_INCLUDE_DIRECTORIES "/nanovg"
  INTERFACE_LINK_LIBRARIES "glfw"
)
```

where `/nanovg` does not exist, for example.  Now (fixed):

```cmake
set_target_properties(nanogui PROPERTIES
  INTERFACE_COMPILE_DEFINITIONS
"NANOGUI_USE_OPENGL;NANOGUI_SHARED;NVG_SHARED"
  INTERFACE_INCLUDE_DIRECTORIES
"${_IMPORT_PREFIX}/include;${_IMPORT_PREFIX}/include/nanovg"
  INTERFACE_LINK_LIBRARIES "glfw"
)
```