Christoph Reiter authored Aug 23, 2018 and Christoph Reiter committed Aug 24, 2018

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.