Drop Xcode 4.1 hack and fix ignored CC var issue
Drop Xcode 4.1 hack and fix ignored CC var issue

Xcode 4.1 was released back in 2011 for Mac OSX 10.6/10.7.
However, OSX 10.7 reached EOL sometime around the end of 2014.
So this `--with-gcc-4.2` hack shouldn't be needed anymore.

Moreover, this patch changes ./configure to honor the CC env-var
again (and thus fix #11231) while giving `--with-gcc=...` a higher
priority over `CC=...`.

So the following 3 invocations are equivalent now:

  CC=... ./configure
  ./configure CC=...
  ./configure --with-gcc=...

Since `--with-{gcc,clang}=...` is a misnomer (as is made apparent by
`--with-gcc=clang` or `--with-clang=gcc`), this would give us a neutral
and idiomatic way to tell ./configure which C compiler to use.

Moreover, `./configure --help` says at the end of its output:

  Some influential environment variables:
    CC		C compiler command
    CFLAGS	C compiler flags
    LDFLAGS	linker flags, e.g. -L<lib dir> if you have libraries in a
		nonstandard directory <lib dir>
    LIBS	libraries to pass to the linker, e.g. -l<library>
    CPPFLAGS	(Objective) C/C++ preprocessor flags, e.g. -I<include dir> if
		you have headers in a nonstandard directory <include dir>
    CPP		C preprocessor

  Use these variables to override the choices made by `configure' or to help
  it to find libraries and programs with nonstandard names/locations.

Consequently, by honoring CC=... (rather than ignoring it) we increase
consistency and reduce user confusion. Ideally, CC=... would become the
recommended way to set the C compiler, and `--with-{clang,gcc}=...`
would be demoted to legacy aliases.

Reviewed By: erikd, bgamari

Differential Revision: https://phabricator.haskell.org/D2046

(cherry picked from commit ffc802e8f617d11de9ece7bed438725bde0300b8)

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