Avoid variable length arrays and remove declarations within code

MSVC doesn't support C99, and building as C++ to be able to use them is
dangerous, as C++ and C99 are incompatible.

Introduce a VARIABLE_ARRAY macro that either uses VLA when supported,
or alloca() otherwise. Note that using alloca() inside loops doesn't
quite work like VLAs, thus the use of VARIABLE_ARRAY there is discouraged.
It might be worth investigating ways to check whether VARIABLE_ARRAY is
used in such context at runtime in debug builds and bail out if that
happens.

include/jemalloc/internal/jemalloc_internal.h.in

@@ -319,6 +319,20 @@ static const bool config_ivsalloc =
 #define	ALIGNMENT_CEILING(s, alignment)					\
 	(((s) + (alignment - 1)) & (-(alignment)))
 
+/* Declare a variable length array */
+#if __STDC_VERSION__ < 199901L
+#  ifdef _MSC_VER
+#    include <malloc.h>
+#    define alloca _alloca
+#  else
+#    include <alloca.h>
+#  endif
+#  define VARIABLE_ARRAY(type, name, count) \
+	type *name = alloca(sizeof(type) * count)
+#else
+#  define VARIABLE_ARRAY(type, name, count) type name[count]
+#endif
+
 #ifdef JEMALLOC_VALGRIND
 /*
  * The JEMALLOC_VALGRIND_*() macros must be macros rather than functions