Do you have to make and install the EnvProcessDll dll separately? If not, you might take the approach of
Math::FFT; in this, there's an XS file (FFT.xs), plus a couple of pure C files (arrays.c and fft4g.c) to supply some needed functions. The
in reply to MakeMaker tailoring
writes a Makefile to compile and link all the files needed to build the extension.
my %opts = (
'NAME' => 'Math::FFT',
'VERSION_FROM' => 'FFT.pm',
'OBJECT' => 'FFT.o fft4g.o arrays.o',