Avoid using the nonstandard "which" command

This command isn't a standard tool; different distributions have
different implementations, and e.g. Fedora doesn't ship it by
default.

Settle on using "command -v $tool >/dev/null" for tests about whether
a tool is available. (We could also use "hash $tool 2>/dev/null" as we
already had in build-all.sh, but settling on using "command -v"
everywhere, for consistency and simplicity.)

build-all.sh

@@ -60,7 +60,7 @@ if [ -z "$PREFIX" ]; then
 fi
 
 for dep in git curl cmake; do
-    if ! hash $dep 2>/dev/null; then
+    if ! command -v $dep >/dev/null; then
         echo "$dep not installed. Please install it and retry" 1>&2
         exit 1
     fi