Makefile supporting variable overriding for tools.

[GoogleCodeExporter]

Use:

SHELL?=$(BIN)/sh
RM?= $(BIN)/rm
AR?= $(BIN)/ar
CC?= $(BIN)/gcc
LD?= $(BIN)/ld 

Instead of:

SHELL=$(BIN)/sh
RM= $(BIN)/rm
AR= $(BIN)/ar
CC= $(BIN)/gcc
LD= $(BIN)/ld

Original issue reported on code.google.com by Stephane.D on 31 Jul 2013 at 7:49

[Stephane-D]

This syntax: SHELL?=$(BIN)/sh

does not seem to be supported by all make tools, at least it does not work on mine. Is there any alternative and more generic syntax for that ?