build: simplify alloca checks. See #1313

The goal of this commit is to reduce differences between the autotools and meson build.

With autotools AC_FUNC_ALLOCA was used which defines HAVE_ALLOCA_H, HAVE_ALLOCA, C_ALLOCA. meson tried to replicate that with has_function() but alloca can be a macro and and is named _alloca under Windows. Since we require a working alloca anyway and only need to know if the header exists replace AC_FUNC_ALLOCA with a simple AC_CHECK_HEADERS.

There is still one user of HAVE_ALLOCA in the embedded gnulib, but since alloca is always provided through galloca.h just force define HAVE_ALLOCA there and add a comment.

The docs were mentioning alloca as an example for cross compiling. Since that variable no longer exists now replace it with another one.