evince: fix build with gcc15

- add "-DHAVE_STRING_H" and "-DHAVE_STDLIB_H" to
`env.NIX_CFLAGS_COMPILE`, similar to what archlinux did in:
https://gitlab.archlinux.org/archlinux/packaging/packages/evince/-/blob/d15c1c165edcb39b3a3a15434a1fdb8ca63b91be/PKGBUILD#L73-74

Alternative would be to patch these declarations out of `kpathsea` source:
https://gitweb.gentoo.org/repo/gentoo.git/plain/dev-libs/kpathsea/files/kpathsea-6.4.0_p20240311-c23.patch?id=d36d6a45aa033cb28dd5850a0fcce501cb9d85a9
or wait for next `texlive` update that includes these fixes:
https://github.com/TeX-Live/texlive-source/commit/cc687b6c0ae8c6a4b566646282a7bc702d822e6d
https://github.com/TeX-Live/texlive-source/commit/4451d937bea57c29b5f9775c5920b9a21a1f140a

Fixes build failure with gcc15:
```
/nix/store/a769c9g0af5g4l4mhh3rcf6ayzmyaakf-texlive-bin-2025-dev/include/kpathsea/c-memstr.h:57:14:
error: conflicting types for 'strtok'; have 'char *(void)'
   57 | extern char *strtok ();
      |              ^~~~~~
In file included from ../backend/dvi/mdvi-lib/paper.c:20:
/nix/store/i1qha7him4faq593wwl1zmhg2pc6lz98-glibc-2.40-66-dev/include/string.h:356:14:
note: previous declaration of 'strtok' with type 'char *(char * restrict,  const char * restrict)'
  356 | extern char *strtok (char *__restrict __s, const char *__restrict __delim)
      |              ^~~~~~
/nix/store/a769c9g0af5g4l4mhh3rcf6ayzmyaakf-texlive-bin-2025-dev/include/kpathsea/c-memstr.h:59:14:
error: conflicting types for 'strstr'; have 'char *(void)'
   59 | extern char *strstr ();
      |              ^~~~~~
/nix/store/i1qha7him4faq593wwl1zmhg2pc6lz98-glibc-2.40-66-dev/include/string.h:350:14:
note: previous declaration of 'strstr' with type 'char *(const char *, const char *)'
  350 | extern char *strstr (const char *__haystack, const char *__needle)
      |              ^~~~~~
/nix/store/a769c9g0af5g4l4mhh3rcf6ayzmyaakf-texlive-bin-2025-dev/include/kpathsea/c-std.h:48:14:
error: conflicting types for 'getenv'; have 'char *(void)'
   48 | extern char *getenv ();
      |              ^~~~~~
In file included from ../backend/dvi/mdvi-lib/tfm.c:20:
/nix/store/i1qha7him4faq593wwl1zmhg2pc6lz98-glibc-2.40-66-dev/include/stdlib.h:773:14:
note: previous declaration of 'getenv' with type 'char *(const char *)'
  773 | extern char *getenv (const char *__name) __THROW __nonnull ((1)) __wur;
      |              ^~~~~~
```

0 files changed, 0 insertions, 0 deletions