Use tabs instead of spaces in Makefile for variables, it doesn't run otherwise.
This also seems to affect your patch files.

GH_TAGNAME isn't needed however upstream suggests that you grab their archive/tarball instead.
https://github.com/premake/premake-core/releases

Use MAKE_CMD instead of hardcoding GMAKE

You might want to consider using a variable instead and patch it in the Makefile rather than hardcoding it.
https://reviews.freebsd.org/D22909 (see the suggestion by mat@)

PORTNAME=premake

and later

PKGNAMESUFFIX= 5

The commit you are using is not for 5.0.0 but for something called v5.0.0-alpha13.

So you should set:

DISTVERSION= 5.0.0-alpha13

and remove the GH_COMMIT variable.

Wrong place in the Makefile. See Chapter 15. Order of Variables in Port Makefiles.

Remove this.

MAKEFILE= Bootstrap.mak
ALL_TARGETS= bsd