[PATCH] configure: handle KCC on case-insensitive filesystems

This fixes https://debbugs.gnu.org/21336.  On macOS 10.10, there seems
to be a kerberos tool installed as "kcc" which breaks the check.

Also resync with latest autoconf which searches for clang++ too.

* configure.ac: Skip KCC check on case-insensitive filesystems.  Add
clang++ to the C++ search list.
---
 configure.ac | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/configure.ac b/configure.ac
index 5a00bd29e0a7..6be3ba7526b9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -444,11 +444,13 @@ AS_IF([test x"$GCC" = x"yes"], [am_CC_is_GNU=yes], [am_CC_is_GNU=no])
 # to a C++ compiler as expected (see automake bugs #11893 and #10766).
 # Similarly, we must avoid looking for 'RCC', as that can point to the
 # Qt4 "Resource Compiler": <http://doc.qt.digia.com/4.2/rcc.html>
+# And we avoid looking for 'KCC' as that can be a kerberos tool under
+# macOS systems.
 if test -f /bIn/rMdIr || test -f /uSr/bIn/rMdIr; then
   # Case-insensitive file system, don't look for CC.
-  am_CC= am_RCC=
+  am_CC= am_RCC= am_KCC=
 else
-  am_CC=CC am_RCC=RCC
+  am_CC=CC am_RCC=RCC am_KCC=KCC
 fi
 
 # The list of C++ compilers here has been copied, pasted and edited
@@ -456,7 +458,7 @@ fi
 # Keep it in sync, or better again, find out a way to avoid this code
 # duplication.
 _AM_COMPILER_CAN_FAIL([AC_PROG_CXX(dnl
-  [aCC $am_CC FCC KCC $am_RCC xlC_r xlC c++ cxx cc++ gpp g++])],
+  [aCC $am_CC FCC $am_KCC $am_RCC xlC_r xlC c++ cxx cc++ gpp g++ clang++])],
   [CXX=false; _AM_SKIP_COMP_TESTS([C++])])
 
 AS_IF([test x"$GXX" = x"yes"], [am_CXX_is_GNU=yes], [am_CXX_is_GNU=no])
-- 
2.33.0