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-libcxx.sh

@@ -55,7 +55,7 @@ LLVM_PATH="$(pwd)/llvm"
 
 cd runtimes
 
-if [ -n "$(which ninja)" ]; then
+if command -v ninja >/dev/null; then
     CMAKE_GENERATOR="Ninja"
     NINJA=1
     BUILDCMD=ninja