libtextstyle: Make gl_INIT_PACKAGE work with Automake >= 1.16.4.
Reported and analyzed by Nicole Mazzuca <mazzucan@outlook.com> in
<https://lists.gnu.org/archive/html/bug-libunistring/2021-08/msg00002.html>.
* m4/init-package-version.m4 (gl_INIT_PACKAGE): Use a new macro gl_INIT_EMPTY.
(gl_INIT_DUMMY): Expand to empty in two steps, via a new macro gl_INIT_DUMMY2.
