vuls needs gmake.

[sheo0147]

On FreeBSD, vuls -v does not work because FreeBSD's make is not gmake.
So, This is change request.

1. Add "Use gmake when compiling vuls." to any documents.
2. Rename all Makefile to GNUmakefile.

I tested on FreeBSD and CentOS6.8 and it worked.

[kotakanbe]

Merged.
Many Thanks.