Modify Makefile.in to work with non-default build locations

When `libnfs` is installed in a non-default location, CPPFLAGS,
LDFLAGS, and LD_LIBRARY_PATH environment variables must be set
when configuring and building dbench. The Makefile should then
use LDFLAGS.
Merge pull request #1 from naegelejd/master

Modify Makefile.in to work with non-default build locations