libtextstyle: Fix the set of exported symbols on Alpine Linux.
* libtextstyle/build-aux/join-v-1: New file, from GNU libunistring.
* libtextstyle/Makefile.am (EXTRA_DIST): Add it.
* libtextstyle/lib/Makefile.am (config.h): Use it instead of 'join -v 1'.
