https://github.com/OpenTTD/OpenGFX/pull/80
--- a/Makefile
+++ b/Makefile
@@ -97,7 +97,7 @@ GREP           ?= grep
 PYTHON         ?= python
 
 # Graphics processing
-GIMP           ?= $(shell which gimp)
+GIMP           ?= $(shell command -v gimp)
 GIMP_FLAGS     ?= -n -i
 
 # NML
@@ -116,7 +116,7 @@ ifdef PNML_FILES
 endif
 
 # GRF tools
-GRFID          ?= $(shell which grfid)
+GRFID          ?= $(shell command -v grfid)
 GRFID_FLAGS    ?= -m
 MUSA           ?= musa.py
 # The license is set via bananas.ini, do not supply a "custom" license.
@@ -606,7 +606,7 @@ endif
 ifeq ($(shell echo "$(OSTYPE)" | cut -d_ -f1),MINGW32)
 # If CC has been set to the default implicit value (cc), check if it can be used. Otherwise use a saner default.
 ifeq "$(origin CC)" "default"
-	CC=$(shell which cc 2>/dev/null && echo "cc" || echo "gcc")
+	CC=$(shell command -v cc 2>/dev/null && echo "cc" || echo "gcc")
 endif
 WIN_VER = $(shell echo "$(OSTYPE)" | cut -d- -f2 | cut -d. -f1)
 ifeq ($(WIN_VER),5)