C source files not allowed when not using cgo

[cmatsuoka]

I'm trying to build a package that imports github.com/tHinqa/outside with --targets=windows/386, and xgo issues the following error when building the imported package that contains a C file. Nothing much special about this setup. I tried with CGO_ENABLED=1 just in case, but it doesn't make any difference. Am I doing something obviously wrong?

Checking docker installation...
Client:
 Version:	18.02.0-ce
 API version:	1.36
 Go version:	go1.9.3
 Git commit:	fc4de44
 Built:	Wed Feb  7 21:16:38 2018
 OS/Arch:	linux/amd64
 Experimental:	false
 Orchestrator:	swarm

Server:
 Engine:
  Version:	18.02.0-ce
  API version:	1.36 (minimum version 1.12)
  Go version:	go1.9.3
  Git commit:	fc4de44
  Built:	Wed Feb  7 21:15:09 2018
  OS/Arch:	linux/amd64
  Experimental:	false

Checking for required docker image karalabe/xgo-latest... found.
Cross compiling agent/cmd/pnm-agent...
Building locally agent/cmd/pnm-agent...
Compiling for windows-4.0/386...
../../../util/dll/dll.go:4:2: C source files not allowed when not using cgo or SWIG: outside.c
2018/02/16 11:35:44 Failed to cross compile package: exit status 1.