Makefile: Change the default compiler from "gcc" to "cc"

Makefile: Change the default compiler from "gcc" to "cc"

Ever since the very first commit to git.git we've been setting CC to
"gcc". Presumably this is behavior that Linus copied from the Linux
Makefile.

However unlike Linux Git is written in ANSI C and supports a multitude
of compilers, including Clang, Sun Studio, xlc etc. On my Linux box
"cc" is a symlink to clang, and on a Solaris box I have access to "cc"
is Sun Studio's CC.

Both of these are perfectly capable of compiling Git, and it's
annoying to have to specify CC=cc on the command-line when compiling
Git when that's the default behavior of most other portable programs.

So change the default to "cc". Users who want to compile with GCC can
still add "CC=gcc" to the make(1) command-line, but those users who
don't have GCC as their "cc" will see expected behavior, and as a
bonus we'll be more likely to smoke out new compilation warnings from
our distributors since they'll me using a more varied set of compilers
by default.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>