The bld file has the sln file specified, but what does it call to build it? MSDev? MSBuild? other?
I want to add some command line params, but I am not sure which executable it calls for unmanaged C++ solutions.
It depends. For Visual Studio 2002/2003, it always calls devenv.com. For Visual Studio 2005 and up, it calls msbuild.exe by default, or devenv or vcbuild if specified in the Override field on the Options tab.
... the action will automatically locate the correct devenv.com or msbuild.exe compiler, based on the version of the project or solution being built. For Visual Studio 2005 and later and Delphi Prism, this action locates and calls msbuild.exe (installed with the .NET Framework 2.0 or later); for Visual Studio 2002/2003, it invokes the appropriate devenv.com compiler for the specified project or solution version.
http://www.kinook.com/VisBuildPro/Manual/vsnetoptionstab.htm