Add alloca() discovery to configure.ac

The build failed on FreeBSD as there is no alloca.h.  This patch is
lifted from the autoconf documentation.

builtin.c

@@ -1,10 +1,22 @@
 #define _BSD_SOURCE
 #define _XOPEN_SOURCE
 #include <sys/time.h>
-#ifdef WIN32
-  #include <malloc.h>
-#else
-  #include <alloca.h>
+#include <stdlib.h>
+#include <stddef.h>
+#ifdef HAVE_ALLOCA_H
+# include <alloca.h>
+#elif !defined alloca
+# ifdef __GNUC__
+#  define alloca __builtin_alloca
+# elif defined _MSC_VER
+#  include <malloc.h>
+#  define alloca _alloca
+# elif !defined HAVE_ALLOCA
+#  ifdef  __cplusplus
+extern "C"
+#  endif
+void *alloca (size_t);
+# endif
 #endif
 #include <assert.h>
 #include <ctype.h>
@@ -13,7 +25,6 @@
 #ifdef HAVE_ONIGURUMA
 #include <oniguruma.h>
 #endif
-#include <stdlib.h>
 #include <string.h>
 #include <time.h>
 #include "builtin.h"

configure.ac

@@ -123,6 +123,8 @@ AM_CONDITIONAL([ENABLE_DOCS], [test "x$enable_docs" != xno])
 AM_CONDITIONAL([ENABLE_ERROR_INJECTION], [test "x$enable_error_injection" = xyes])
 AM_CONDITIONAL([ENABLE_ALL_STATIC], [test "x$enable_all_static" = xyes])
 
+AC_FUNC_ALLOCA
+
 AC_FIND_FUNC([isatty], [c], [#include <unistd.h>], [0])
 AC_FIND_FUNC([_isatty], [c], [#include <io.h>], [0])
 AC_FIND_FUNC([strptime], [c], [#include <time.h>], [0, 0, 0])

util.c

@@ -10,8 +10,24 @@
 #include <fcntl.h>
 #include <limits.h>
 #include <string.h>
-#include <stdlib.h>
 #include <unistd.h>
+#include <stdlib.h>
+#include <stddef.h>
+#ifdef HAVE_ALLOCA_H
+# include <alloca.h>
+#elif !defined alloca
+# ifdef __GNUC__
+#  define alloca __builtin_alloca
+# elif defined _MSC_VER
+#  include <malloc.h>
+#  define alloca _alloca
+# elif !defined HAVE_ALLOCA
+#  ifdef  __cplusplus
+extern "C"
+#  endif
+void *alloca (size_t);
+# endif
+#endif
 #ifndef WIN32
 #include <pwd.h>
 #endif