Fix build break on *BSD

Linux uses alloca.h; many other operating systems define alloca(3) in
stdlib.h.

Signed-off-by: Garrett Cooper <yanegomi@gmail.com>
diff --git a/configure.ac b/configure.ac
index 88297c5..aee7482 100644
--- a/configure.ac
+++ b/configure.ac
@@ -261,6 +261,7 @@
 	CFLAGS="$CFLAGS"
 	CPPFLAGS="$CPPFLAGS -D_GNU_SOURCE"
 	abi="elf"
+	AC_DEFINE([JEMALLOC_HAS_ALLOCA_H])
 	AC_DEFINE([JEMALLOC_PURGE_MADVISE_DONTNEED], [ ])
 	AC_DEFINE([JEMALLOC_THREADED_INIT], [ ])
 	JEMALLOC_USABLE_SIZE_CONST=""