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.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/test/test-udev.c b/src/test/test-udev.c
index 7a4622b875..ab31f5a2f1 100644
--- a/src/test/test-udev.c
+++ b/src/test/test-udev.c
@@ -11,6 +11,7 @@
 #include <sys/signalfd.h>
 #include <unistd.h>
 
+#include "build.h"
 #include "device-private.h"
 #include "fs-util.h"
 #include "log.h"
@@ -81,7 +82,7 @@ static int run(int argc, char *argv[]) {
                 return 0;
         }
 
-        log_debug("version %s", PACKAGE_VERSION);
+        log_debug("version %s", GIT_VERSION);
         mac_selinux_init();
 
         action = argv[1];