clar unit testing framework FTBFS on uclibc systems (wchar_t unsupported)

Hi,

Since clar unit testing framework was imported by commit 9b7caa2809cb (t: import the clar unit testing framework, 2024-09-04), Git FTBFS on uclibc systems built by Buildroot:

```
    CC t/unit-tests/unit-test.o
t/unit-tests/clar/clar.c: In function 'clar__assert_equal':
t/unit-tests/clar/clar.c:767:23: error: unknown type name 'wchar_t'
  767 |                 const wchar_t *wcs1 = va_arg(args, const wchar_t *);
      |                       ^~~~~~~
In file included from t/unit-tests/clar/clar.c:13:
t/unit-tests/clar/clar.c:767:58: error: unknown type name 'wchar_t'
  767 |                 const wchar_t *wcs1 = va_arg(args, const wchar_t *);
      |                                                          ^~~~~~~
t/unit-tests/clar/clar.c:768:23: error: unknown type name 'wchar_t'
  768 |                 const wchar_t *wcs2 = va_arg(args, const wchar_t *);
      |                       ^~~~~~~
t/unit-tests/clar/clar.c:768:58: error: unknown type name 'wchar_t'
  768 |                 const wchar_t *wcs2 = va_arg(args, const wchar_t *);
      |                                                          ^~~~~~~
t/unit-tests/clar/clar.c:769:65: warning: implicit declaration of function 'wcscmp' [-Wimplicit-function-declaration]
  769 |                 is_equal = (!wcs1 || !wcs2) ? (wcs1 == wcs2) : !wcscmp(wcs1, wcs2);
      |                                                                 ^~~~~~
t/unit-tests/clar/clar.c:784:23: error: unknown type name 'wchar_t'
  784 |                 const wchar_t *wcs1 = va_arg(args, const wchar_t *);
      |                       ^~~~~~~
t/unit-tests/clar/clar.c:784:58: error: unknown type name 'wchar_t'
  784 |                 const wchar_t *wcs1 = va_arg(args, const wchar_t *);
      |                                                          ^~~~~~~
t/unit-tests/clar/clar.c:785:23: error: unknown type name 'wchar_t'
  785 |                 const wchar_t *wcs2 = va_arg(args, const wchar_t *);
      |                       ^~~~~~~
t/unit-tests/clar/clar.c:785:58: error: unknown type name 'wchar_t'
  785 |                 const wchar_t *wcs2 = va_arg(args, const wchar_t *);
      |                                                          ^~~~~~~
t/unit-tests/clar/clar.c:787:65: warning: implicit declaration of function 'wcsncmp' [-Wimplicit-function-declaration]
  787 |                 is_equal = (!wcs1 || !wcs2) ? (wcs1 == wcs2) : !wcsncmp(wcs1, wcs2, len);
      |                                                                 ^~~~~~~
make[1]: *** [Makefile:2795: t/unit-tests/clar/clar.o] Error 1
```

See [1] for the full build log.

Thanks.

[1]: https://autobuild.buildroot.org/results/8cc9795dc18277926dd386eb1cb9f8c9b65b0042/build-end.log