Detect OpenMP in configure when requested.

By default, OpenMP support is disabled.
add lattice openmp
parallelize operators
parallelize FD utilities
Add OpenMP flags to quantlib-config.
Merge pull request #45.

Initial OpenMP support from Joseph Wang.  The support is disabled by
default; use `configure --enable-openmp` to enable it.
At this time, only a few simple loops in the FD and tree modules are
parallelized with a `#pragma omp parallel for` directive.