Fix build errors on BSD (in BSDMakefile) (#27594)

Fix build errors on BSD (in BSDMakefile) (#27594)

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. ```