build: read package version from a top-level VERSION file

The version string lived only in configure.ac's AC_INIT. Move it to a
neutral VERSION file (line 1: numeric X.Y.Z.W, line 2: optional suffix)
so a second build system can read the same source. AC_INIT now joins
the two lines via m4_esyscmd_s; the resulting PACKAGE_VERSION is
unchanged ("2.4.8.4 (Prelim)").

Author: matejk

VERSION

@@ -0,0 +1,2 @@
+2.4.8.4
+(Prelim)

configure.ac

@@ -36,7 +36,12 @@
 dnl
 dnl LTFS configure.ac.
 dnl
-AC_INIT([LTFS], [2.4.8.4 (Prelim)], IBM corporation.)
+dnl Package version is kept in the top-level VERSION file (line 1: numeric
+dnl X.Y.Z.W, line 2: optional suffix) so the autotools and CMake builds share
+dnl a single source of truth. The awk joins them into "X.Y.Z.W suffix".
+AC_INIT([LTFS],
+    m4_esyscmd_s([awk 'NR==1{n=$0} NR==2{s=$0} END{if(s=="")printf "%s",n; else printf "%s %s",n,s}' VERSION]),
+    IBM corporation.)
 AC_CONFIG_SRCDIR([src/main.c])
 AC_CONFIG_AUX_DIR([build-aux])
 AC_CONFIG_MACRO_DIRS([m4])