[SCons] Add support to import custom variables from parent SConstruct

[adamscott]

Makes parent SConstruct able to apply their custom.py values to the main SConstruct file.

Otherwise, even if the parent SConstruct has use_llvm = True, the main SConstruct file will ignore it unless you manually add use_llvm=yes in the command line.

[dsnopek]

Thanks!

However, since the parent SConstruct can already pass in it's own env, is this really needed? Like, couldn't you do something like:

env = Environment(tools=["default"], PLATFORM="")
Export('env')
SConscript("godot-cpp/SConstruct")

This seems to overlap with what was added in PR #1170

[adamscott]

Closing that PR, will do another one.

[adamscott]

Thanks!

However, since the parent SConstruct can already pass in it's own env, is this really needed? Like, couldn't you do something like:

env = Environment(tools=["default"], PLATFORM="")
Export('env')
SConscript("godot-cpp/SConstruct")

I tested out and it doesn't work. What custom.py sets up is variables to be picked up by opts.Add commands. Then, the value truly retained by scons is not the variables set by "custom.py", but the options set.

So, when we are passing env, it doesn't actually contain the values of the local "custom.py", hence the need to pass "customs" as an exported value.