improve clang support (6)

[Bruno Haible]

clang supports __builtin_alloca.
On Windows, it does not have an <alloca.h> header file, and therefore the use
of alloca(...) without any header file produces a warning
  warning: implicitly declaring library function 'alloca' with type 'void 
*(unsigned long long)'

Therefore, the bestes choices for gnulib are
  - to use __builtin_alloca, or
  - to use _alloca from <malloc.h>.
Both produce identical code. But I find it more convenient to use a
compiler built-in rather than an MSVC header file.


2020-08-07  Bruno Haible  <bruno@clisp.org>

        Use __builtin_alloca with clang.
        * lib/alloca.in.h (alloca): Define as __builtin_alloca on clang.

diff --git a/lib/alloca.in.h b/lib/alloca.in.h
index beb022c..8d9ae06 100644
--- a/lib/alloca.in.h
+++ b/lib/alloca.in.h
@@ -44,7 +44,7 @@
 # endif
 #endif
 #ifndef alloca
-# ifdef __GNUC__
+# if defined __GNUC__ || (__clang_major__ >= 4)
 #  define alloca __builtin_alloca
 # elif defined _AIX
 #  define alloca __alloca