meson: generate version tag from git

$ build/systemctl --version
systemd 239-3555-g6178cbb5b5
+PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN2 +IDN +PCRE2 default-hierarchy=hybrid
$ git tag v240 -m 'v240'
$ ninja -C build
ninja: Entering directory `build'
[76/76] Linking target fuzz-unit-file.
$ build/systemctl --version
systemd 240
+PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN2 +IDN +PCRE2 default-hierarchy=hybrid

This is very useful during development, because a precise version string is
embedded in the build product and displayed during boot, so we don't have to
guess answers for questions like "did I just boot the latest version or the one
from before?".

This change creates an overhead for "noop" builds. On my laptop, 'ninja -C
build' that does nothing goes from 0.1 to 0.5 s. It would be nice to avoid
this, but I think that <1 s is still acceptable.

Fixes #7183.

PACKAGE_VERSION is renamed to GIT_VERSION, to make it obvious that this is the
more dynamically changing version string.

Why save to a file? It would be easy to generate the version tag using
run_command(), but we want to go through a file so that stuff gets rebuilt when
this file changes. If we just defined an variable in meson, ninja wouldn't know
it needs to rebuild things.

src/basic/build.h

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
+#include "version.h"
+
 #if HAVE_PAM
 #define _PAM_FEATURE_ "+PAM"
 #else

src/basic/meson.build

@@ -1,5 +1,10 @@
 # SPDX-License-Identifier: LGPL-2.1+
 
+version_h = vcs_tag(
+        command: vcs_tagger,
+        input : 'version.h.in',
+        output : 'version.h')
+
 basic_sources = files('''
         MurmurHash2.c
         MurmurHash2.h

src/basic/util.c

@@ -557,7 +557,7 @@ uint64_t system_tasks_max_scale(uint64_t v, uint64_t max) {
 }
 
 int version(void) {
-        puts("systemd " PACKAGE_VERSION "\n"
+        puts("systemd " GIT_VERSION "\n"
              SYSTEMD_FEATURES);
         return 0;
 }

src/basic/version.h.in

@@ -0,0 +1 @@
+#define GIT_VERSION "@VCS_TAG@"