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.

configure.ac

@@ -477,7 +477,6 @@ AS_IF([test "$glib_native_win32" = "yes"], [
 AM_CONDITIONAL(OS_WIN32_AND_DLL_COMPILATION, [test x$glib_native_win32 = xyes -a x$glib_win32_static_compilation != xyes])
 
 # Checks for library functions.
-AC_FUNC_ALLOCA
 AC_CHECK_FUNCS(mmap posix_memalign memalign valloc fsync pipe2 issetugid)
 AC_CHECK_FUNCS(timegm gmtime_r posix_spawn)
 AC_FUNC_STRERROR_R()
@@ -641,6 +640,7 @@ AC_CHECK_HEADERS([sys/vfs.h sys/vmount.h sys/statfs.h sys/statvfs.h sys/filio.h
 AC_CHECK_HEADERS([mntent.h sys/mnttab.h sys/vfstab.h sys/mntctl.h fstab.h])
 AC_CHECK_HEADERS([linux/magic.h])
 AC_CHECK_HEADERS([termios.h])
+AC_CHECK_HEADERS([alloca.h])
 
 # Some versions of MSC lack these
 AC_CHECK_HEADERS([dirent.h sys/time.h])
@@ -2985,10 +2985,8 @@ _______EOF
 # on variable expansion in case labels.  Look at the generated config.status
 # for a hint.
 
-if test "x${ac_cv_working_alloca_h+set}" = xset ; then
-  glib_header_alloca_h="$ac_cv_working_alloca_h"
-else
-  glib_header_alloca_h="$ac_cv_header_alloca_h"
+if test x$ac_cv_header_alloca_h = xyes ; then
+  glib_header_alloca_h=yes
 fi
 
 if test x$enable_included_printf = xyes ; then