curl-compilers.m4: detect xlclang

Since it isn't totally clang compatible, we detect this IBM clang
front-end and if detected, avoids some clang specific magic.

Reported-by: Kees Dekker
Fixes #3474
Closes #3476

m4/curl-compilers.m4

@@ -5,7 +5,7 @@
#                            | (__| |_| |  _ <| |___
#                             \___|\___/|_| \_\_____|
#
# Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
# Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
#
# This software is licensed as described in the file COPYING, which
# you should have received as part of this distribution. The terms
@@ -83,7 +83,16 @@ AC_DEFUN([CURL_CHECK_COMPILER_CLANG], [
  CURL_CHECK_DEF([__clang__], [], [silent])
  if test "$curl_cv_have_def___clang__" = "yes"; then
    AC_MSG_RESULT([yes])
    compiler_id="CLANG"
    AC_MSG_CHECKING([if compiler is xlclang])
    CURL_CHECK_DEF([__xlc__], [], [silent])
    if test "$curl_cv_have_def___xlc__" = "yes" ; then
      dnl IBM's almost-compatible clang version
      AC_MSG_RESULT([yes])
      compiler_id="XLCLANG"
    else
      AC_MSG_RESULT([no])
      compiler_id="CLANG"
    fi
    fullclangver=`$CC -v 2>&1 | grep version`
    clangver=`echo $fullclangver | grep "based on LLVM " | "$SED" 's/.*(based on LLVM \(@<:@0-9@:>@*\.@<:@0-9@:>@*\).*)/\1/'`
    if test -z "$clangver"; then