build: Replace which command with command -v

This change made in a way that is compatible with GNU Make versions
older than 4.3.

depends/hosts/mingw32.mk

@@ -1,4 +1,4 @@
-ifneq ($(shell which $(host)-g++-posix),)
+ifneq ($(shell $(SHELL) $(.SHELLFLAGS) "command -v $(host)-g++-posix"),)
 mingw32_CXX := $(host)-g++-posix
 endif