travis-ci: build with the right compiler

Our 'Makefile' hardcodes the compiler to build Git as 'CC = cc'.  This
CC variable 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 whatever
'cc' happens to be on the platform.

Our build jobs on Travis CI are badly affected by this.  In the build
matrix we have dedicated build jobs to build Git with GCC and Clang
both on Linux and macOS from the very beginning (522354d70f (Add
Travis CI support, 2015-11-27)).  Alas, this never really worked as
supposed to, because Travis CI specifies the compiler for those build
jobs as 'export CC=gcc' and 'export CC=clang' (which works fine for
projects built with './configure && make').  Consequently, our
'linux-clang' build job has always used GCC, because that's where 'cc'
points at in Travis CI's Linux images, while the 'osx-gcc' build job
has always used Clang.  Furthermore, 37fa4b3c78 (travis-ci: run gcc-8
on linux-gcc jobs, 2018-05-19) added an 'export CC=gcc-8' in an
attempt to build with a more modern compiler, but to no avail.

Set MAKEFLAGS with CC based on the $CC environment variable, so 'make'
will run the "right" compiler.  The Xcode 10.1 macOS image on Travis
CI already contains the gcc@8 package from Homebrew, but we have to
'brew link' it first to be able to use it.

So with this patch our build jobs will build Git with the following
compiler versions:

  linux-clang: clang version 5.0.0 (tags/RELEASE_500/final)
  linux-gcc:   gcc-8 (Ubuntu 8.1.0-5ubuntu1~14.04) 8.1.0

  osx-clang: Apple LLVM version 10.0.0 (clang-1000.11.45.5)
  osx-gcc:   gcc-8 (Homebrew GCC 8.2.0) 8.2.0

  GETTEXT_POISON: gcc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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