Date::Pcalc -> Date::Calc

Date::Calc is nowadays pure perl (but may use Date::Calc::XS if available), so
there's no need anymore to use Date::Pcalc (which does not work since
perl 5.16 anyway).
basic.t: BBBikeApacheSessionCountedHandler skipped if mod_perl2 not available

Note that the skip check is only active if BBBIKE_TEST_NO_CGI_TESTS
was set.
workaround for "make: don't know how to make w. Stop"

This problem happens if the outer make is GNU make, and there's at
least one recursive make invocation. In this case GNU make sets the
MAKEFLAGS environment variable to "w". But some test scripts may call
freebsd-make or pmake, and the content of the MAKEFLAGS variable is
interpreted as the (non-existing) target w.

Fix is to unset the MAKEFLAGS variable before calling freebsd-make,
pmake, or BSD's native make.