ifstat: fix build with GCC 15

ifstat: fix build with GCC 15

ifstat fails to build with GCC 15 due to an incorrect detection of the
signal handler return type.

The configure script shipped with ifstat uses an obsolete K&R-style
declaration:

  void (*signal())();

With GCC 15 this causes the signal() return type check to incorrectly
assume 'int' instead of 'void'. As a result, the generated code defines
signal handlers returning int, which conflicts with struct sigaction
expecting a void (*)(int) handler and leads to a build failure:

  assignment to 'void (*)(int)' from incompatible pointer type
  'int (*)(int)'

Enable autoreconf during the build to regenerate the configure
script, which correctly detects the signal handler type and fixes
the compilation error.

Signed-off-by: Til Kaiser <redacted>