SwiftPM: How to setup Swift module.map referring to two connected C libraries

Question

I'm trying to build a Swift Package Manager system package (a module.modulemap) making available two system C libraries where one includes the other.

That is, one (say libcurl) is a base module and the other C library is including that (like so: #include "libcurl.h"). On the regular C side this works, because the makefiles pass in proper -I flags and all is good (and I could presumably do the same in SPM, but I'd like to avoid extra flags to SPM).

So what I came up with is this module map:

module CBase [system] {
  header "/usr/include/curl.h"
  link "curl"
  export *
}

module CMyLib [system] {
  use CBase
  header "/usr/include/mylib.h"
  link "mylib"
  export *
}

I got importing CBase in a Swift package working fine. But when I try to import CMyLib, the compiler complains:

error: 'curl.h' file not found

Which is kinda understandable because the compiler doesn't know where to look (though I assumed that use CBase would help).

Is there a way to get this to work w/o having to add -Xcc -I flags to the build process?

Update 1: To a degree this is covered in Swift SR-145 and SE-0063: SwiftPM System Module Search Paths. The recommendation is to use the Package.swift pkgConfig setting. This seems to work OK for my specific setup. However, it is a chicken and egg if there is no .pc file. I tried embedding an own .pc file in the package, but the system package directory isn't added to the PKG_CONFIG_PATH (and hence won't be considered during the compilation of a dependent module). So the question stands: how to accomplish that in an environment where there libs are installed, but w/o a .pc file (just header and lib).