Fix failing commands

[sryze]

This fixes two errors:

1. The invocation of gmake always fails due to || and && having the same precedence (missing parentheses):

$ make -de
/usr/local/bin/gmake

*** Failed target:  .BEGIN
*** Failed command: which "gmake" || printf "Error: GNU Make is required!\n\n" 1>&2 && false
*** Error code 1

Stop.

2. gmake complains that it's being passed an empty argument when $(GPREFIX) is empty due to the enclosing quotation marks, removing them fixes it:

$ make -de
/usr/local/bin/gmake
gmake: *** empty string invalid as file name.  Stop.

*** Failed target:  .DONE
*** Failed command: "gmake" "" "--no-print-directory" 
*** Error code 2

Stop.

I was able to build Gitea (https://github.com/go-gitea/gitea) successfully with these changes on current versions of FreeBSD/NetBSD/OpenBSD. I'm not sure if this breaks something on older BSD versions as I'm not very familiar with BSDs (if so, please let me know).