www/mathopd: Fix build with default PIE

Mathopd Makefile passed CFLAGS in MAKE_ENV variable. This caused CFLAGS
processing in bsd.prog.mk to be ignored, which in turn left non-PIE CFLAGS
from sys.mk and LDFLAGS with PIE enabled from bsd.prog.mk.

PR: 253275 [exp-run]
Submitted by: Dawid Górecki <dgr@semihalf.com>
Approved by: lwhsu, Michiel Boland [maintainer]
Differential Revision: https://reviews.freebsd.org/D28793