devel/argtable: Fix missing .pc file

[ultima]

Summary

• Fix overly-broad patch on Makefile to add missing .pc file
• Cleanup Makefile

PR\: 221210
Submitted by\: Yuri Victorovich (maintainer)
Reviewed by\: lifanov (mentor), matthew (mentor)
Approved by\: lifanov (mentor), matthew (mentor)
MFH: 2017Q3
Differential Revision\: https://reviews.freebsd.org/DXXXXX

Test Plan

portlint:
WARN: Makefile: for new port, make $FreeBSD$ tag in comment section empty, to make SVN happy.
WARN: Makefile: new ports should not set PORTREVISION.
WARN: Makefile: new ports should not be maintained by [e-mail].
0 fatal errors and 3 warnings found.

poudriere:
103i386
103amd64
110i386
110amd64
12i386
12amd64

[matthew]