More fixes for compilers that reject K&R function definitions.

More fixes for compilers that reject K&R function definitions.

This fixes all of the remaining failures exposed by running the
testsuite with GCC 12 and
CC='cc -Wimplicit-function-declaration -Wold-style-definition
       -Wimplicit-int -Werror'
.

* lib/autoconf/c.m4 (_AC_C_C89_TEST_GLOBALS): Don’t use K&R function
  definitions.
* lib/autoconf/specific.m4 (AC_SYS_RESTARTABLE_SYSCALLS): Likewise.
  (AC_FUNC_MKTIME): Declare functions taking no arguments as ‘fn (void)’
  not ‘fn ()’.
* lib/autoconf/c.m4 (_AC_C_C99_TEST_GLOBALS): Declare free().