[PostGIS] #6092: fuzzingengine doesn't use hardening CPPFLAGS

[PostGIS]

#6092: fuzzingengine doesn't use hardening CPPFLAGS
----------------------------+---------------------------
 Reporter:  Bas Couwenberg  |      Owner:  strk
     Type:  defect          |     Status:  new
 Priority:  medium          |  Milestone:  PostGIS 3.7.0
Component:  build           |    Version:  master
 Keywords:                  |
----------------------------+---------------------------
 The blhc CI job failed for the Debian package build:
 {{{
 5635:CPPFLAGS missing (-D_FORTIFY_SOURCE=2): cc -g -O2 -Werror=implicit-
 function-declaration -ffile-prefix-map=/build/package/package=. -fstack-
 protector-strong -fstack-clash-protection -Wformat -Werror=format-security
 -fcf-protection -c -o fuzzingengine.o
 /build/package/package/fuzzers/fuzzingengine.c
 }}}
 https://salsa.debian.org/debian-gis-team/postgis/-/jobs/9911778

 The attached patch fixes the issue by using `CPPFLAGS` in addition to
 `CFLAGS`.
-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/6092>
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