[Rd] alloca() on FreeBSD (PR#7890)

[vangyzen at stat.duke.edu]

Full_Name: Eric van Gyzen
Version: 2.1.0
OS: FreeBSD 5.4
Submission from: (NULL) (152.3.22.33)


R-2.1.0 fails to compile on the newest release of FreeBSD, complaining about
undefined references to __builtin_alloca.  On FreeBSD, alloca() is declared in
stdlib.h, not alloca.h as the R sources expect.  Therefore, HAVE_DECL_ALLOCA
does not get set, so the R sources declare alloca() after it has already been
#defined by stdlib.h.

A possible fix is to modify the configure script to look for the declaration of
alloca() in stdlib.h (perhaps after it fails to find it in alloca.h).

This was previously reported in PR#7881.