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.

src/tsd.c

@@ -36,7 +36,7 @@ JEMALLOC_ATTR(visibility("default"))
 void
 _malloc_thread_cleanup(void)
 {
-	bool pending[ncleanups], again;
+	bool pending[MALLOC_TSD_CLEANUPS_MAX], again;
 	unsigned i;
 
 	for (i = 0; i < ncleanups; i++)