community/darktable: fix segfault on start

06e71650 (community/darktable: build with -O2 to fix build,
2021-07-28) tried to fix the segfault, but setting CXXFLAGS does not
work.

Setting CFLAGS does suffice, so set -O2 via CFLAGS.
8 jobs for !23703 with darktable-fix-crash in 4 minutes and 17 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Lint
passed #449522
docker-alpine x86_64
lint

00:00:18

 
  Build
passed #449527
docker-alpine ci-build aarch64
build-aarch64

00:03:58

passed #449529
armhf docker-alpine ci-build
build-armhf

00:00:27

passed #449528
docker-alpine ci-build armv7
build-armv7

00:00:27

passed #449526
docker-alpine ci-build ppc64le
build-ppc64le

00:00:48

passed #449525
docker-alpine ci-build s390x
build-s390x

00:00:25

passed #449524
docker-alpine ci-build x86
build-x86

00:00:16

passed #449523
docker-alpine ci-build x86_64
build-x86_64

00:01:24