Derive configs

A config can be derived from another config in Project.meta like this:

ExecutableConfig A
LibraryConfig    B, extends: A
CustomConfig     C, extends: B
ExecutableConfig D, extends: C

The config type of the parent config does not matter, but only settings which are valid in BOTH configs will be inherited.
In the example above D gets the dependencies from A, because "Dependency" is valid in all configs, but D does not get the "Files" from A, because "Files" is not valid in CustomConfig.

The following listing shows HOW settings will be derived:

Setting	Derived
Dependency	parent + child
ExternalLibrary ExternalLibrarySearchPath UserLibrary	parent + child (ordered by line number!)
PreSteps	parent + child
PostSteps	parent + child
Toolchain DefaultToolchain	Compiler, Archiver and Linker settings used from parent if not existing in child - see below
Makefile/CommandLine (in CustomConfig)	used from parent if not in child
Files	parent + child
ExcludeFiles	parent + child
IncludeDir	parent + child
LinkerScript	used from parent if not in child
ArtifactName	used from parent if not in child
MapFile	used from parent if not in child

Example for deriving toolchains:

ExecutableConfig A {
  Toolchain {
    Compiler CPP {
      Define "DefA"
    }
    Linker {
      Flags "-O3"
    }
  }
}

ExecutableConfig B, extends: A {
  Toolchain {
    Compiler CPP {
      Define "DefB1"
    }
    Compiler ASM {
      Define "DefB2"
    }
    Archiver {
      Flags "-B"
    }
  }
}

B derives from A, but B redefines Compiler CPP, therefore "DefA" will not be defined in B. But the toolchain of B does not include a Linker tag, therefore the Linker will be inherited.
The toolchain of B is treated like this:

Toolchain {
  Compiler CPP {
    Define "DefB1"
  }
  Compiler ASM {
    Define "DefB2"
  }
  Archiver {
    Flags "-B"
  }
  Linker {
    Flags "-O3"
  }
}