build: Remove the need for an autoconf test.
* gettext-runtime/intl/intl-exports.c (IMP): Use predefined preprocessor symbol
tests, not USER_LABEL_PREFIX_UNDERSCORE.
* gettext-runtime/configure.ac (INTL_EXPORTS_FLAGS): Remove variable.
* gnulib-local/modules/gettext-runtime-misc (Makefile.am): Don't use
INTL_EXPORTS_FLAGS.
* gettext-tools/woe32dll/export.h (IMP): Use predefined preprocessor symbol
tests, not USER_LABEL_PREFIX_UNDERSCORE.
* gettext-tools/configure.ac (GETTEXTLIB_EXPORTS_FLAGS): Don't define USER_LABEL_PREFIX_UNDERSCORE.
