`buildscripts/buildall.sh`: `declare -g` doesn't work in bash 3.2

3. buildscripts/buildall.sh: declare -g doesn't work in bash 3.2

What happened: CI failed with:

./buildall.sh: line 31: declare: -g: invalid option

Root cause: markbuilt() used declare -g "$varname=0" to set a dynamically-named global variable. declare -g requires bash 4.2+. macOS ships bash 3.2 as its system /bin/bash (Apple stopped updating bash for GPLv3 licensing reasons around that version), and GitHub's macos-14 runner invokes workflow steps with that same system bash unless a script explicitly re-execs itself under a newer one.

Fix: replaced with eval "$varname=0", which achieves the same dynamic-variable-assignment effect and works identically on bash 3.2 and newer.

Lesson: any bash feature added to this project's scripts needs to be checked against bash 3.2 compatibility, not just "does this work on my own machine" — a developer's personal machine likely has a newer Homebrew-installed bash that CI does not use by default.