fix: fix a compilation error on GCC/Linux

    xstrdup.c:13:18: warning: implicit declaration of function ‘strdup’; did you mean ‘xstrdup’? [-Wimplicit-function-declaration]
       13 |     char *copy = strdup(str);
          |                  ^~~~~~
          |                  xstrdup

As Stack Overflow says:

> It allows you to use functions that are not part of the standard C
> library but are part of the POSIX.1 (IEEE Standard 1003.1) standard.
> Using the macros described in feature_test_macros allows you to
> control the definitions exposed by the system header files.
>
> As far as I know _POSIX_SOURCE is obsolete and you should use
> _POSIX_C_SOURCE instead.
>
> For example, if you want to use strndup, you have to use
>
> #define _POSIX_C_SOURCE 200809L

https://stackoverflow.com/a/48332467/2103996

And the libc manual:

> Macro: _POSIX_C_SOURCE
>
> Define this macro to a positive integer to control which POSIX
> functionality is made available. The greater the value of this macro,
> the more functionality is made available.
>
> If you define this macro to a value greater than or equal to 1, then
> the functionality from the 1990 edition of the POSIX.1 standard (IEEE
> Standard 1003.1-1990) is made available.
>
> If you define this macro to a value greater than or equal to 2, then
> the functionality from the 1992 edition of the POSIX.2 standard (IEEE
> Standard 1003.2-1992) is made available.
>
> If you define this macro to a value greater than or equal to 199309L,
> then the functionality from the 1993 edition of the POSIX.1b standard
> (IEEE Standard 1003.1b-1993) is made available.
>
> If you define this macro to a value greater than or equal to 199506L,
> then the functionality from the 1995 edition of the POSIX.1c standard
> (IEEE Standard 1003.1c-1995) is made available.
>
> If you define this macro to a value greater than or equal to 200112L,
> then the functionality from the 2001 edition of the POSIX standard
> (IEEE Standard 1003.1-2001) is made available.
>
> If you define this macro to a value greater than or equal to 200809L,
> then the functionality from the 2008 edition of the POSIX standard
> (IEEE Standard 1003.1-2008) is made available.

https://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html

lua/wincent/commandt/lib/xstrdup.c

@@ -5,6 +5,8 @@
 
 #include "xstrdup.h"
 
+#define _POSIX_C_SOURCE 200809L /* needed in order to see strdup() */
+
 #include <stddef.h> /* for NULL */
 #include <stdlib.h> /* for abort() */
 #include <string.h> /* for strdup() */