wolfssl: fix build with make < 4.2

[ptpt52]

Fixes: #11866

[robimarko]

@cotequeiroz

Cannot $(shell) be used to evaluate the script as external file?

[cotequeiroz]

I'm on vacation now, so I won't be able to try it for a while. Testing is just s matter of comparing the script.
My original proposal was to inline the script, so if it does not work, you have another option. I don't think I still have the original commit, but it shouldn't be too bad.

There's one thing I would like to avoid: using makefile syntax inside a separate shell script file.

Edit: I've just realized this is inline the script.

[hauke]

Thank you for the patch, I added it to master in fcde517.
I extended the description a bit.

We should probably backport it to stable versions too.