patch to improve portability of configure.in

This patch improves the portability of configure.in using standard autoconf friendly variables.

This patch, from NetBSD's pkgsrc tree, makes it easier to build and install the package on non-Linux platforms.

It makes sure to bring in the dynamic library loading autoconf module for imporoved portability;
It makes sure to bring in an automake CC module, again, for improved portability;
It changes a number of hard-coded filesystem paths into autoconf shell variables of the same meaning;