make interfaces/perl5 build properly outside source directory

This is a 2 line patch to src/interfaces/perl5/GNUMakefile that fixes
the 'override CPPFLAGS' to include the source directory during compile,
and makes the install target look in the proper place for the man page.

Changes are only required when building outside the source directory.

--
J. R. Nield
jrnield@usol.com