On overriding make variables from the environment...

Our Makefile has lines like these:

  CFLAGS = -g -O2 -Wall
  CC = cc
  AR = ar
  SPATCH = spatch

Note the use of '=', not '?='. This means that these variables can be overridden from the command line, i.e. 'make CC=gcc-X.Y' will build with that particular GCC version, but not from the environment, i.e. 'CC=gcc-X.Y make' will still build with 'cc'.

This can be confusing for developers who come from other projects where they used to run 'CC=whatever make'.

And our build jobs on Travis CI are badly affected by this. We have dedicated build jobs to build Git with GCC and Clang both on Linux and OSX from the very beginning (522354d70f (Add Travis CI support, 2015-11-27)). But guess how Travis CI specifies which compiler to use! With 'export CC=gcc' and 'export CC=clang', respectively. Consequently, our Clang Linux build job has always used gcc, because that's where 'cc' points at on Linux by default, while the GCC OSX build job has always used Clang. Oh, well. Furthermore, see 37fa4b3c78 (travis-ci: run gcc-8 on linux-gcc jobs, 2018-05-19), where Duy added an 'export CC=gcc-8' in an attempt to use a more modern compiler, but this had no effect either.

I'm not sure what to do. I'm fine with updating our 'ci/' scripts to explicitly respect CC in the environment (either by running 'make CC=$CC' or by writing $CC into 'config.mak'). Or I could update our Makefile to use '?=' for specific variables, but:

  - I'm afraid to break somebody's setup relying on the current
    behavior and CC having different values in the environment and in
    'config.mak'.

  - Where to stop, IOW which variables should be set with '?='?
    CFLAGS, LDFLAGS, CC, AR, ..., SPATCH, SPATCH_FLAGS?  Dunno.  We
    already have 'STRIP ?= strip' and there are variables that are
    checked explicitly (e.g. 'DEVELOPER=y make' works).

    Note also that prior to b05701c5b4 (Make CFLAGS overridable from
    make command line., 2005-08-06) our Makefile used 'CC?=gcc' as
    well.