avoid building unitprotos.h at lib build-time

[bagder]

I did this

buildprotos.h requires perl and is built in the lib directory at build-time, adding perl as a build prerequisite even when building from a tarball, which we try to avoid.

Ref: #17750 (comment)

I expected the following

Either we ship unitprotos.h in the tarball or we generate it first when building the tests.

curl/libcurl version

git master

operating system

any

[bagder]

Oh, @vszakats! We should make our build-from-tarball CI test hide/uninstall perl so that we better detect if we introduce a perl dependency for this again.

[vszakats]

Oh, @vszakats! We should make our build-from-tarball CI test hide/uninstall perl so that we better detect if we introduce a perl dependency for this again.

That'd be a good idea.

I almost never test without Perl, and just realized that cmake builds ignore
the pre-built hugehelp.c in both in-tree and out-of-tree builds. Also
something to test in CI maybe. Once fixed, but my initial attempts weren't
successful.

[vszakats]

→ #18104