bug#6170: 24.0.50; Compiling on solaris2.10 with gcc doesn't define alloca

[Lawrence Mitchell <wence@gmx.li>]

On this system, <stdlib.h> is provided by Sun and therefore
doesn't define alloca, unlike on a typical GNU/linux system where
<stdlib.h> contains the following:

| #if defined __USE_GNU || defined __USE_BSD || defined __USE_MISC
| # include <alloca.h>
| #endif /* Use GNU, BSD, or misc.  */

When compiling emacs with gcc, alloca is therefore undefined.
The culprit is this snippet in configure.in:

| #ifndef __GNUC__
| # ifdef HAVE_ALLOCA_H
| #  include <alloca.h>
| # else /* AIX files deal with #pragma.  */
| #  ifndef alloca /* predefined by HP cc +Olibcalls */
| char *alloca ();
| #  endif
| # endif /* HAVE_ALLOCA_H */
| #endif /* __GNUC__ */

On this system, configure correctly defines both HAVE_ALLOCA_H
and HAVE_ALLOCA, but since the inclusion is guarded by __GNUC__
not being defined, we never include <alloca.h>.

I think the correct fix is to unconditionally define alloca if
__GNUC__ is detected, see patch below.  Note that this is closer
to how autoconf checks for the presence of the alloca function.

Possible changelog entry:

2010-05-11  Lawrence Mitchell  <wence@gmx.li>

	* configure.in: Unconditionally define alloca if __GNUC__ is
	detected.

diff --git a/configure.in b/configure.in
index 8a7d9be..98b6abb 100644
--- a/configure.in
+++ b/configure.in
@@ -3360,6 +3360,9 @@ extern char *getenv ();
 char *alloca ();
 #  endif
 # endif /* HAVE_ALLOCA_H */
+#else
+# undef alloca
+# define alloca __builtin_alloca
 #endif /* __GNUC__ */
 #ifndef HAVE_SIZE_T
 typedef unsigned size_t;