Avoid a crash when using musl

[matttbe]

Without this PR, a warning is emitted when compiling libyaml with musl libc:

api.c: In function 'yaml_strdup':
api.c:66:27: warning: implicit declaration of function 'strdup'; did you mean 'strcmp'? [-Wimplicit-function-declaration]
     return (yaml_char_t *)strdup((char *)str);
                           ^~~~~~
                           strcmp

If the warning is ignored (default behaviour), a program using libyaml starts but crashes because strdup() is returning garbage:

(gdb) b parser.c:1365
Breakpoint 1 at 0x43ecf4: file parser.c, line 1365.
(gdb) run
(...)
Breakpoint 1, yaml_parser_append_tag_directive (
    parser=parser@entry=0x7ffffff5f0, value=...,
    allow_duplicates=allow_duplicates@entry=1, mark=...) at parser.c:1365
1365    in parser.c
(gdb) p copy
$1 = {
  handle = 0xffffffffb8000fe0 <error: Cannot access memory at address 0xffffffffb8000fe0>,
  prefix = 0xffffffffb7f76fe0 <error: Cannot access memory at address 0xffffffffb7f76fe0>}

The solution is to follow strdup()'s manpage and define _GNU_SOURCE before including string.h.

I guess when using other libc, it fallbacks to another version or maybe _GNU_SOURCE is set by default but it certainly uses a valid strdup.

Fixes: 625fcfe ("Refactor internal and external API.")

[matttbe]

It looks like the error reported by the CI is not due to this modification:

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