autoconf grammar has been changed and the configure.ac file can be corrected by autoupdate.

➜ diff configure.ac configure.ac~
2c2
< AC_INIT([tcltls],[1.8.0])
---
> AC_INIT(tcltls, 1.8.0)
8c8
< AC_USE_SYSTEM_EXTENSIONS
---
> AC_GNU_SOURCE
256,257c256
< AC_CONFIG_FILES([Makefile pkgIndex.tcl tcltls.syms])
< AC_OUTPUT
---
> AC_OUTPUT(Makefile pkgIndex.tcl tcltls.syms)