How to run a C filter at build time?

[John Gardiner Myers]

I'm finding the MakeMaker documentation inpenetrable.  Here's what I'm 
trying to do:

The source to my Perl module has two files, Flock.pm.in and lockdefs.c.  
To build, it needs to compile lockdefs.c with the C compiler, then run 
the generated executable with Flock.pm.in as input, generating 
Flock.pm.  The generated Flock.pm is what is installed.  The C 
executable is not installed.

lockdefs needs to be in C as it has to obtain information from the 
system include files.