Revert "support/dependencies: don't check for `which'"

This reverts commit a95c353a46247db036862ad839c095ca9ae4f690.

Switching to using 'command -v' instead of 'which', opened a can of
worms that is hard to fix in a timely manner:

  - recursive call to 'make' from a post-build, post-iamge script, fails
    because of a redefinition of HOSTCC_NOCCACHE (a bug on its own that
    needs a separate fix anyway) [0];

  - 'make' believeing it can call "simple" commands with execve() et al.
    instead of passing them through a shell via system(), and thus
    failing to find 'command' in the PATH [1].

[0] https://lore.kernel.org/buildroot/20211001175329.GA1973888@lbrmn-mmayer.ric.broadcom.net/T/#m95c17eb8374e4e3dd6eee700d397aa12cca0739e
[1] https://lore.kernel.org/buildroot/20211001180304.GV1504958@scaer/T/#m3a8f36bd76ec7d8e5038a6c8932bb6ffe23ea268

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

support/dependencies/dependencies.sh

@@ -65,6 +65,8 @@ check_prog_host()
 	fi
 }
 
+# Verify that which is installed
+check_prog_host "which"
 # Verify that sed is installed
 check_prog_host "sed"