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

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