windows-spawn: Improve GCC 11 allocation-deallocation checking

[Bruno Haible]

2021-08-07  Bruno Haible  <bruno@clisp.org>

        windows-spawn: Improve GCC 11 allocation-deallocation checking.
        * lib/windows-spawn.h: Include <stdlib.h>.
        (compose_command, compose_envblock): Declare that deallocation must
        happen through 'free'.

diff --git a/lib/windows-spawn.h b/lib/windows-spawn.h
index 78c7736..5ba4664 100644
--- a/lib/windows-spawn.h
+++ b/lib/windows-spawn.h
@@ -20,6 +20,7 @@
 
 #include <stdbool.h>
 #include <stdint.h>
+#include <stdlib.h>
 
 /* Get declarations of the native Windows API functions.  */
 #define WIN32_LEAN_AND_MEAN
@@ -72,14 +73,16 @@ extern const char ** prepare_spawn (const char * const 
*argv,
    prepare_spawn.
    Returns a freshly allocated string.  In case of memory allocation failure,
    NULL is returned, with errno set.  */
-extern char * compose_command (const char * const *argv);
+extern char * compose_command (const char * const *argv)
+  _GL_ATTRIBUTE_MALLOC _GL_ATTRIBUTE_DEALLOC_FREE;
 
 /* Composes the block of memory that contains the environment variables.
    ENVP must contain an environment (a NULL-terminated array of string of the
    form VARIABLE=VALUE).
    Returns a freshly allocated block of memory.  In case of memory allocation
    failure, NULL is returned, with errno set.  */
-extern char * compose_envblock (const char * const *envp);
+extern char * compose_envblock (const char * const *envp)
+  _GL_ATTRIBUTE_MALLOC _GL_ATTRIBUTE_DEALLOC_FREE;
 
 
 /* This struct keeps track of which handles to pass to a subprocess, and with