Here is the entire Makefile.PL for that distribution:

use 5.006;
use ExtUtils::MakeMaker;
# See lib/ExtUtils/MakeMaker.pm for details of how to influence
# the contents of the Makefile that is written.
WriteMakefile(
    NAME              => 'Math::GammaFunction',
    VERSION_FROM      => 'lib/Math/GammaFunction.pm', # finds $VERSION
    PREREQ_PM         => {}, # e.g., Module::Name => 1.1
    LICENSE => 'perl',
    ($] >= 5.005 ?     ## Add these new keywords supported since 5.005
      (ABSTRACT_FROM  => 'lib/Math/GammaFunction.pm', # retrieve abstr
+act from module
       AUTHOR         => 'Steffen Mueller <smueller@cpan.org>') : ()),
    LIBS              => ['-lRmath', '-lm'], # e.g., '-lm'
    DEFINE            => '', # e.g., '-DHAVE_SOMETHING'
    INC               => '-I.', # e.g., '-I. -I/usr/include/other'
    #INC               => '-I. -I/usr/share/R/include', # e.g., '-I. -
+I/usr/include/other'
    # Un-comment this if you add C files to link with later:
    # OBJECT            => '$(O_FILES)', # link all the C files too
);
[download]

I'm not going to install the module, but if I were going to install it with an alternate location for -lRmath, I might try seeing what happens when I make the following change:

    INC               => '-I. -I/home/me/include',
[download]

...where /home/me/include is the path to my local copy of the library. Give it a try, and pay attention to any error messages it generates, because they might tell you if you've specified one level too deep, or not deep enough on the route to identifying the proper path.

Since you already suggested we assist with the distribution's Makefile.PL, I'm wondering if you already looked at the file, and tried what I'm suggesting.