Fix for perl issue building cli_wallet on Ubuntu 20 with PERL5LIB in environment

[xloem]

This patch resolves an issue I was getting on Ubuntu 20 trying to build cli_wallet:

[ 90%] Generating api_documentation.cpp
Can't locate doxygen/perlmod/DoxyDocs.pm in @INC (you may need to install the doxygen::perlmod::DoxyDocs module) (@INC contains: /home/karl/perl5/lib/perl5/x86_64-linux-gnu-thread-multi /home/karl/perl5/lib/perl5 /home/karl/perl5/lib/perl5/x86_64-linux-gnu-thread-multi /home/karl/perl5/lib/perl5 /home/karl/perl5/lib/perl5/x86_64-linux-gnu-thread-multi /home/karl/perl5/lib/perl5 /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.30.0 /usr/local/share/perl/5.30.0 /usr/lib/x86_64-linux-gnu/perl5/5.30 /usr/share/perl5 /usr/lib/x86_64-linux-gnu/perl/5.30 /usr/share/perl/5.30 /usr/local/lib/site_perl /usr/lib/x86_64-linux-gnu/perl-base) at /home/karl/src/trading-setup/bitshares-core/libraries/wallet/generate_api_documentation.pl line 6.
make[3]: *** [libraries/wallet/CMakeFiles/graphene_wallet.dir/build.make:63: libraries/wallet/api_documentation.cpp] Error 2

Perl doesn't seem to be picking up the PERLLIB environment variable. I checked the perl manpage and it suggests using -I to add to @INC, not PERLLIB. This patch changs the build file to do that, which resolves the above issue for me.

[abitmore]

Thanks.

Please rebase to develop branch.

The PERLLIB environment variable was introduced in #479 and was updated in #1539. I'm not sure if we need different commands for different versions of Perl.

[xloem]

@abitmore a quick fix for your perl version concern would be to set both the environment variable and the passed argument; does doing that sound okay to you?

[xloem]

I've rebased to develop. It looks like the reason I ran into this was local to my system: I have PERL5LIB set and under man perlrun it states that PERL5LIB overrides any PERLLIB. I think -I is the way to go because it preserves the user's environment if they have a nonstandard install, but switching to PERL5LIB would also have prevented this quirk.

[abitmore]

Thanks for the update.

Looks like #2085 is the same issue, but it's on Ubuntu 18. Both of you have perl5 in your home directory. Not sure if he has PERL5LIB environment variable set though.

[xloem]

Sorry for the repost. #2085 looks like the exact same thing: something, likely PERL5LIB, took priority over PERLLIB to add the home folder directories. ./ should also be robust against local environment issues, maybe moreso.