Have syntax custom_gdnative=../godot-cpp

**Describe the project you are working on:**

Multiplayer 3d game

**Describe the problem or limitation you are having in your project:**

Using multiple gdnative libraries means each has a copy of godot-cpp. This causes problems with ensuring that they use the same version of godot-cpp.

**Describe the feature / enhancement and how it helps to overcome the problem or limitation:**

Add `custom_gdnative` or `custom_godot_cpp` scons option to the template. The name is arbitrary.

**Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:**

The new scons option will do a search outside of the current repository.

**If this enhancement will not be used often, can it be worked around with a few lines of script?:**

Yes. A few lines of scons :D

**Is there a reason why this should be core and not an add-on in the asset library?:**

Build system things for gdnative aren't core.