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-make.sh

@@ -45,7 +45,7 @@ PREFIX="$(cd "$PREFIX" && pwd)"
 : ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64}}
 
 download() {
-    if [ -n "$(which wget)" ]; then
+    if command -v wget >/dev/null; then
         wget "$1"
     else
         curl -LO "$1"