package/cvs: fix mktime related compile failure

package/cvs: fix mktime related compile failure

Use ac_cv_func_working_mktime=yes to force the use of a provided
mktime implementation instead of compiling the failing own one.

Fixes:

  http://autobuild.buildroot.net/results/5bcd8f4235002da682cc900f866116d2fe87f1c8

  mktime.c: In function 'ydhms_diff':
  mktime.c:106:52: error: size of array 'a' is negative
   #define verify(name, assertion) struct name { char a[(assertion) ? 1 : -1]; }
                                                      ^
  mktime.c:170:3: note: in expansion of macro 'verify'
     verify (long_int_year_and_yday_are_wide_enough,
     ^~~~~~

with the failure/assert comming from the lines:

  verify (long_int_year_and_yday_are_wide_enough,
          INT_MAX <= LONG_MAX / 2 || TIME_T_MAX <= UINT_MAX);

which fails since the y2038 time_t conversion from 32bit to 64bit
(musl libc).

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>