AC_PROG_CC wrongly setting $GCC=yes while clang is used

[Bastien Chevreux]

Hello there,

having used gcc for ages, I decided to try and play around with clang for one 
of my C/C++ projects. Unfortunately, it looks as if autoconf 2.69 gets things 
horribly wrong with clang.

I’m literally forcing it to find only clang, but it still insists on trying to 
tell me I’m running a gcc. In my configure.ac I have:

  …
  AC_PROG_CC([clang])
  echo "I have dollarGCC= $GCC"
  exit

which results in the following output:

  checking for clang... clang
  checking whether the C compiler works... yes
  checking for C compiler default output file name... a.out
  checking for suffix of executables... 
  checking whether we are cross compiling... no
  checking for suffix of object files... o
  checking whether we are using the GNU C compiler... yes
  checking whether clang accepts -g... yes
  checking for clang option to accept ISO C89... none needed
  checking whether clang understands -c and -o together... yes
  checking for style of include used by make... GNU
  checking dependency style of clang... gcc3
  I have dollarGCC= yes

I’m not sure how to continue from here. I looked at the config log, but only 
see the following:

  configure:3334: checking for clang
  configure:3350: found /usr/bin/clang
  configure:3361: result: clang
  configure:3392: checking for C compiler version
  configure:3401: clang --version >&5
  Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
  Target: x86_64-apple-darwin13.3.0
  Thread model: posix
  configure:3412: $? = 0
  configure:3401: clang -v >&5
  Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
  Target: x86_64-apple-darwin13.3.0
  Thread model: posix
  configure:3412: $? = 0
  configure:3401: clang -V >&5
  clang: error: argument to '-V' is missing (expected 1 value)
  clang: error: no input files
  configure:3412: $? = 1
  configure:3401: clang -qversion >&5
  clang: error: unknown argument: '-qversion' 
[-Wunused-command-line-argument-hard-error-in-future]
  clang: note: this will be a hard error (cannot be downgraded to a warning) in 
the future
  clang: error: no input files
  configure:3412: $? = 1
  configure:3432: checking whether the C compiler works
  configure:3454: clang -mmacosx-version-min=10.6     conftest.c  >&5
  configure:3458: $? = 0
  configure:3506: result: yes
  configure:3509: checking for C compiler default output file name
  configure:3511: result: a.out
  configure:3517: checking for suffix of executables
  configure:3524: clang -o conftest -mmacosx-version-min=10.6     conftest.c  
>&5
  configure:3528: $? = 0
  configure:3550: result: 
  configure:3572: checking whether we are cross compiling
  configure:3580: clang -o conftest -mmacosx-version-min=10.6     conftest.c  
>&5
  configure:3584: $? = 0
  configure:3591: ./conftest
  configure:3595: $? = 0
  configure:3610: result: no
  configure:3615: checking for suffix of object files
  configure:3637: clang -c -mmacosx-version-min=10.6    conftest.c >&5
  configure:3641: $? = 0
  configure:3662: result: o
  configure:3666: checking whether we are using the GNU C compiler
  configure:3685: clang -c -mmacosx-version-min=10.6    conftest.c >&5
  configure:3685: $? = 0
  configure:3694: result: yes

Am I doing something wrong and if yes, any idea how this can be fixed?

Best,
  Bastien