Replace the manual construction of ${LOCALBASE}/lib/${CC}, which is not
appropriate for clang and fragile for GCC, by ${LDFLAGS}.

This is not only more robust, ${LDFLAGS} will also provide -Wl,-rpath
when necessary.

Plus extend an existing CONFIGURE_ENV instead of overwriting it.

With these changes, the configure problem that made the build fail
goes away. On FreeBSD 10.x and above there still are C++ errors
later in the build, so leave the port BROKEN.

Approved by: swills (maintainer)
With help from: redports.org, mat, decke