fix: add ac_cv_type_time_t=yes for gnulib cross-compilation

Without this cache variable, gnulib's configure incorrectly assumes
time_t is unavailable when cross-compiling for Redox, generating
broken fallback headers that fail with 'time_t undeclared' at
the compile-time integrality check.

local/recipes/dev/bison/recipe.toml

@@ -56,6 +56,7 @@ export ac_cv_type_ssize_t=yes
 export ac_cv_type_ptrdiff_t=yes
 export ac_cv_type_nlink_t=yes
 export ac_cv_type_mbstate_t=yes
+export ac_cv_type_time_t=yes
 export gl_cv_type_intmax_t=yes
 export gl_cv_type_ptrdiff_t_signed=yes
 export gl_cv_header_inttypes_h_with_uintmax=yes

local/recipes/dev/m4/recipe.toml

@@ -65,6 +65,7 @@ export ac_cv_type_ssize_t=yes
 export ac_cv_type_ptrdiff_t=yes
 export ac_cv_type_nlink_t=yes
 export ac_cv_type_mbstate_t=yes
+export ac_cv_type_time_t=yes
 export ac_cv_type_sigset_t=yes
 export ac_cv_type_posix_spawnattr_t=yes
 export ac_cv_type_posix_spawn_file_actions_t=yes