PHP fails to build with an error: ext/gd/gd.lo is not a valid libtool object

Got stuck with PHP 7.1 upgrade on Directadmin server? Seeing an error of the following kind:

libtool: link: `ext/gd/gd.lo' is not a valid libtool object
make: *** [sapi/cli/php] Error 1
make: *** Waiting for unfinished jobs....
libtool: link: `ext/gd/gd.lo' is not a valid libtool object
make: *** [sapi/fpm/php-fpm] Error 1
libtool: link: `ext/gd/gd.lo' is not a valid libtool object
make: *** [sapi/phpdbg/phpdbg] Error 1
libtool: link: `ext/gd/gd.lo' is not a valid libtool object
make: *** [sapi/cgi/php-cgi] Error 1

Here is what worked for us, and it might work for you too:

Clean everything:

First you need to clean custombuild directory from old source files of PHP. 

It was noticed that custombuild script does not remove directories with source files of PHP 7.x when you simply instruct custombuild script to clean everything. So here we remove possibly existing ./php-*/ folders. It's OK to see an error if you don't have any folder with that name.

cd /usr/local/directadmin/custombuild/
./build clean
rm -rf ./php*/

Update custombuild script:

We need to make sure that you have the latest version of custombuild script and versions.txt is actual too.

cd /usr/local/directadmin/custombuild/
./build update

Install Libtool:

Yes, we run this command as your version might be too old, and we need to make sure you've got the latest available.

cd /usr/local/directadmin/custombuild/
./build libtool

Build PHP:

And finally we install or upgrade PHP with the following commands:

cd /usr/local/directadmin/custombuild/
./build php

Done

If everything went fine you should see:

PHP 7.1.9 Installed.

That's it! Good luck ;)