Skip building XS when PUREPERL_ONLY=1 set

[zmughal]

This change skips the static/dynamic sections of the Makefile when using a
pure-Perl install so that no XS linking is done at all.

Compare the build logs below:

$ for br in master pureperl-skip-linking; do \
echo "On branch $br"; \
git checkout -q $br ; \
git clean -f; \
( perl Makefile.PL PUREPERL_ONLY=1 && make && make clean)  | grep -Fe $( perl -MConfig -e 'print $Config{cc}' ); \
echo; \
done

On branch master
Checking for cc... cc
cc -c   -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2 -O2   -DVERSION=\"1.102\" -DXS_VERSION=\"1.102\" -fPIC "-I$HOME/perl5/perlbrew/perls/perl-5.26.1/lib/5.26.1/x86_64-linux/CORE"   Util.c
cc  -shared -O2 -L/usr/local/lib -fstack-protector-strong  Util.o  -o blib/arch/auto/Params/Util/Util.so  \

On branch pureperl-skip-linking
Checking for cc... cc

[zmughal]

I see there is not a CI configuration running any more (previously Travis CI). Would it help to use GitHub Actions to test this out?

[rehsack]

I would love a PR which test whether this provides a reasonable and working change :)
Further, I'd prefer commit messages which explain the business case behind a commit, not the technical change. Most people reading commits can derive technical stuff on it's own and the remaining need the business reasons.

[zmughal]

@rehsack, OK, sounds good!

You can review updated commits to see if this fits the requirements. A workflow that tests what I'm getting at is here.

[zmughal]

Hi, if you have time, could you review this. It should be working on multiple platforms per the new CI workflow.

[zmughal]

@rehsack, just reaching out again about this PR.

[zmughal]

@rehsack, hello, do you have a moment to review this PR?

[zmughal]

Hello @rehsack, I just ran the CI I made here again and it still passes. I also added a few more Perl versions to test against.