1. `make build` fails because `||` and `&&` have the same precedence in
sh/bash, so the `false` command always evaluated (leading to an error).
```
+ which gmake /usr/local/bin/gmake
+ false
*** Failed target: .BEGIN
*** Failed command: which "gmake" || printf "Error: GNU Make is
required!\n\n" 1>&2 && false
*** Error code 1
```
2. When `GPREFIX` is set to an empty string with quotation marks,
`gmake` mistakenly thinks that it's a file name:
``` gmake: *** empty string invalid as file name. Stop. ```
When compiling GItea for Linux on Windows, you get a `gitea.exe` file as
output, but because it's a Linux executable, the `.exe` extension is
unnecessary.
This PR adds a check for `GOOS` environment variable in addition to
`OS`.
Sergey ZolotarevandGitHub