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>