Properties for defining macros or variables

For a Visual Studio project I have a .props file that defines certain configuration macros that are used at other places using the $(macroname) syntax.

I found premake's externalrule for including external property files. However, this not only tries to include the external .props file, but also a .target file (which doesn't exist in my case). And it seems to be bound to handling files of a certain type.

Is there a way to generate the necessary import syntax for an external .props file containing generic macros? If yes, how? If no, is it planned to add such a feature to premake?

A similar mechanism would be useful for generated makefiles (gmake). In makefiles you can define variables like FOO ?= foo and can override the value of FOO by adding something like FOO=bar as a command line parameter on invoking make. Is this supported by premake? If yes, how? If no, is it planned to add such a feature?

The issue names no source files or tests. Start by tracing premake's externalrule handling for Visual Studio projects and its gmake generator, then determine whether generic property imports and command-line-overridable variables already have entry points. Done means documenting existing support or defining the scope and generator-specific behavior for a new feature.