build: Modernize autoconf macros.
* gettext-runtime/m4/intdiv0.m4 (gt_INTDIV0): Improve m4 quoting.
* gettext-runtime/m4/printf-posix.m4 (gt_PRINTF_POSIX): Likewise.
* gettext-runtime/m4/intl.m4 (AM_INTL_SUBDIR): Improve indentation.
* gettext-tools/m4/hostname.m4 (gt_PREREQ_HOSTNAME): Improve m4 quoting. Use
AC_CACHE_CHECK and AC_COMPILE_IFELSE.
* gettext-tools/m4/siginfo.m4 (gt_SIGINFO): Improve m4 quoting. Use
AC_COMPILE_IFELSE.
