Do not include <alloca.h> unconditionally

[arichardson]

Summary

Some systems (e.g. FreeBSD) do not provide this header. This commit fixes
building the testsuite on FreeBSD after c4c6c75c1ff888f47a203e5aeadc85b1d2aaa172

[ro]

For good measure tested on sparcv9-sun-solaris2.11, amd64-pc-solaris2.11, and x86_64-pc-linux-gnu.

LGTM.

Sorry for the breakage.

MultiSource/Applications/oggenc/oggenc.c

 /* OggEnc
  *
  * This program is distributed under the GNU General Public License, version 2.
  * A copy of this license is included with this source.
  *
  * Copyright 2000-2002, Michael Smith <[e-mail]>
  *
  * Portions from Vorbize, (c) Kenneth Arnold <[e-mail]>
  * and libvorbis examples, (c) Monty <[e-mail]>
  */
+#if __has_include(<alloca.h>)
 #include <alloca.h>
+#endif
 #include <assert.h>
 #include <ctype.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <locale.h>
 #include <math.h>
 #include <stdarg.h>
 #include <stdio.h>

MultiSource/Benchmarks/MiBench/consumer-lame/rtp.c

 #include <stdlib.h>
 #include <string.h>
 #include <netinet/in.h>
 #include <unistd.h>
 #include <stdlib.h>
 #include <stdio.h>
+#if __has_include(<alloca.h>)
 #include <alloca.h>
+#endif
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <arpa/inet.h>
 
 struct rtpbits {
   int sequence:16;  /* sequence number: random */
   int pt:7; /* payload type: 14 for MPEG audio */
   int m:1;  /* marker: 0 */