Feature request: subdir option for cmake module?

[ethindp]

Would it be possible to add a subdir kwarg to cmake.subproject to specify where a project's CMakeLists.txt file is? This would greatly simplify using CMake projects that have non-conventional project structures. The idea is that subdir would take a path relative to the root of the extracted source tree, so even '.' could be used if you wanted to explicitly tell meson that the CMake project is at the root.