Use "gmake" command in custommake plugin on FreeBSD. Fixes unit test.

Authored by arrowd on Aug 10 2018, 1:50 PM.

Description

Use "gmake" command in custommake plugin on FreeBSD. Fixes unit test.

Reviewers: KDevelop

Subscribers: kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D14731