Gnulib's alloca.h used even when there is a system header

[Eli Zaretskii]

Building the latest version of Texinfo, I see that Gnulib decides to
generate its own alloca.h header even though there's a system header
alloca.h (and HAVE_ALLOCA_H is 1 in config.h).  Why does it do that?
Isn't that dangerous? e.g., it could conflict with how the system
header defies 'alloca', no?