grub-mkconfig: Use portable "command -v" to detect installed programs

The "which" utility is not guaranteed to be installed either, and if it
is, its behavior is not portable either.

Conversely, the "command -v" shell builtin is required to exist in all
POSIX 2008 compliant shells, and is thus guaranteed to work everywhere.

Examples of open-source shells likely to be installed as /bin/sh on
Linux, which implement the 11-year-old standard: ash, bash, busybox,
dash, ksh, mksh and zsh.

A side benefit of using the POSIX portable option is that it requires
neither an external disk executable, nor (because unlike "which", the
exit code is reliable) a subshell fork. This therefore represents a mild
speedup.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

util/grub-mkconfig_lib.in

@@ -34,7 +34,7 @@ if test "x$grub_mkrelpath" = x; then
   grub_mkrelpath="${bindir}/@grub_mkrelpath@"
 fi
 
-if which gettext >/dev/null 2>/dev/null; then
+if command -v gettext >/dev/null; then
   :
 else
   gettext () {