use ExtUtils::MakeMaker;
# See lib/ExtUtils/MakeMaker.pm for details of how to influence
# the contents of the Makefile that is written.
WriteMakefile(
    'NAME'	=> 'Foo',
    'VERSION_FROM' => 'Foo.pm', # finds $VERSION
defined $ENV{'HACK'} ? (
    'LIBS'	=> ['-L.. -lhack'],   # e.g., '-lm' 
) : (
    'LIBS'	=> [''],   # e.g., '-lm' 
),
defined $ENV{'HACK'} ? (
    'DEFINE'	=> '-DUSE_EXTERN',     # e.g., '-DHAVE_SOMETHING'
) : (),
    'INC'	=> '',     # e.g., '-I/usr/include/other' 
);