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-python.sh

@@ -48,7 +48,7 @@ if [ -z "$CHECKOUT_ONLY" ]; then
 fi
 
 MAKE=make
-if [ -n "$(which gmake)" ]; then
+if command -v gmake >/dev/null; then
     MAKE=gmake
 fi