CXXFLAGS not always used during build leads to missing hardening features

Steps to reproduce

$ curl -o icmake-13.02.00.tar.gz https://gitlab.com/fbb-git/icmake/-/archive/13.01.00/icmake-13.01.00.tar.gz
$ tar -xf icmake-13.02.00.tar.gz
$ cd icmake-13.02.00/icmake/
$ export CXXFLAGS="-march=x86-64 -mtune=generic -O2 -pipe -fno-plt -fexceptions -Wp,-D_FORTIFY_SOURCE=3 -Wformat -Werror=format-security -fstack-clash-protection -fcf-protection -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -Wp,-D_GLIBCXX_ASSERTIONS -g -ffile-prefix-map=/build/icmake/src=/usr/src/debug/icmake -flto=auto"
$ ./prepare /
$ ./buildlib /
$ ICMAKE_CPPSTD="-std=c++26" ./build all
$ readelf -n tmp/usr/bin/icmake 

Displaying notes found in: .note.gnu.property
  Owner                Data size 	Description
  GNU                  0x00000030	NT_GNU_PROPERTY_TYPE_0
      Properties: x86 ISA needed: x86-64-baseline
	x86 feature used: x86, XMM
	x86 ISA used: x86-64-baseline

Displaying notes found in: .note.gnu.build-id
  Owner                Data size 	Description
  GNU                  0x00000014	NT_GNU_BUILD_ID (unique build ID bitstring)
    Build ID: ab1ced5ec649451d06c4074d224e5ed90a866d82

Displaying notes found in: .note.ABI-tag
  Owner                Data size 	Description
  GNU                  0x00000010	NT_GNU_ABI_TAG (ABI version tag)
    OS: Linux, ABI: 4.4.0

expected:

readelf -n tmp/usr/bin/icmake 

Displaying notes found in: .note.gnu.property
  Owner                Data size 	Description
  GNU                  0x00000040	NT_GNU_PROPERTY_TYPE_0
      Properties: x86 feature: IBT, SHSTK
	x86 ISA needed: x86-64-baseline
	x86 feature used: x86, XMM
	x86 ISA used: x86-64-baseline

Displaying notes found in: .note.gnu.build-id
  Owner                Data size 	Description
  GNU                  0x00000014	NT_GNU_BUILD_ID (unique build ID bitstring)
    Build ID: 0b9eef60a73c36aa5435e73c5d4757a0f07acda0

Displaying notes found in: .note.ABI-tag
  Owner                Data size 	Description
  GNU                  0x00000010	NT_GNU_ABI_TAG (ABI version tag)
    OS: Linux, ABI: 4.4.0

Note the missing x86 feature: IBT, SHSTK. I believe this is due to CXXFLAGS not being passed to g++ in icmake/buildscripts/multicomp and icmake/buildscripts/precomp.