Document required build tool: which (JuliaLang#37494)

On minimal Linux environments `which` may not be an installed tool. Alternatively `command -v` could be used as an alternative in the Makefiles to avoid this dependency.

doc/build/build.md

@@ -156,6 +156,7 @@ Building Julia requires that the following software be installed:
 - **[cmake]** (>= 3.4.3) — needed to build `libgit2`.
 - **[pkg-config]** — needed to build `libgit2` correctly, especially for proxy support.
 - **[powershell]** (>= 3.0) — necessary only on Windows.
+- **[which]** — needed for checking build dependencies.
 
 On Debian-based distributions (e.g. Ubuntu), you can easily install them with `apt-get`:
 ```
@@ -220,6 +221,7 @@ uses are listed in [`deps/Versions.make`](https://github.com/JuliaLang/julia/blo
 [mbedtls]: https://tls.mbed.org/
 [pkg-config]: https://www.freedesktop.org/wiki/Software/pkg-config/
 [powershell]: https://docs.microsoft.com/en-us/powershell/scripting/wmf/overview
+[which]: https://carlowood.github.io/which/
 
 ## Build dependencies