There is no such thing as += unless you happen to be using the Bash programming language, i.e. your script shebang is /bin/bash.
However, configure scripts are run via /bin/sh instead, which may or may not be be bash, usually depending on whether the system in question preferred to have one less package installed (in which case it is bash) vs. have a faster /bin/sh installed (in which case it is probably the dash shell).
There is no such thing as
+=unless you happen to be using the Bash programming language, i.e. your script shebang is /bin/bash.However, configure scripts are run via /bin/sh instead, which may or may not be be bash, usually depending on whether the system in question preferred to have one less package installed (in which case it is bash) vs. have a faster /bin/sh installed (in which case it is probably the dash shell).