library not found for -lX11

Hi,

I submitted a package to CRAN:
http://cran.r-project.org/web/packages/vimcom/index.html

It builds on Linux without warnings, but there is the following error
on Mac OS X:

    ld: library not found for -lX11
    collect2: ld returned 1 exit status
    make: *** [vimcom.so] Error 1
    ERROR: compilation failed for package 'vimcom'

The complete log is here:
http://www.r-project.org/nosvn/R.check/r-release-macosx-ix86/vimcom-00install.html

I know that I have to edit the src/Makevars file, but I don't know
what I should put there and I don't have access to any machine running
Mac OS X to make tests. Any help will be appreciated.

Thanks,

Jakson Alves de Aquino
Federal University of Cear?
Social Sciences Department
www.lepem.ufc.br/aquino.php