lang/fpc-units: Finally fix default build on FreeBSD 9

On FreeBSD9, libEGL is built by GCC which requires binutils as a run
depends. fpc-cairo requires libEGL. fpc-libbfd and binutils conflict
with each other due to both installing the same header. Thus, on
FreeBSD 9, the BFD and CAIRO options cannot coexist. Since both were
set on by default, no binary package for fpc-units has built for months.

Since there is no mechanism to set options by release, I use bmake's
exist() function to check for /usr/include/lwres which only exists on
FreeBSD 9. If it's present, the BFD option is disabled by default. This
should restore the building of the fpc-units package on FreeBSD 9.

Reported by: pkg-fallout (for months)