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.
   ```