build-sys: use -Og instead of -O0 to catch warnings

  $ touch src/core/dbus.c; make CFLAGS=-O0
  make --no-print-directory all-recursive
  Making all in .
    CC       src/core/libsystemd_core_la-dbus.lo
    CCLD     libsystemd-core.la

  $ touch src/core/dbus.c; make CFLAGS=-Og
  make --no-print-directory all-recursive
  Making all in .
    CC       src/core/libsystemd_core_la-dbus.lo
  src/core/dbus.c: In function 'init_registered_system_bus':
  src/core/dbus.c:798:18: warning: 'id' may be used uninitialized in this function [-Wmaybe-uninitialized]
           dbus_free(id);
                    ^
    CCLD     libsystemd-core.la

-Og Optimize debugging experience. -Og enables optimizations that do
not interfere with debugging. It should be the optimization level of
choice for the standard edit-compile-debug cycle, offering a
reasonable level of optimization while maintaining fast compilation
and a good debugging experience.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/core/machine-id-setup.c b/src/core/machine-id-setup.c
index 18e015fe7f..1b55da7e56 100644
--- a/src/core/machine-id-setup.c
+++ b/src/core/machine-id-setup.c
@@ -153,7 +153,7 @@ static int generate(char id[34]) {
 int machine_id_setup(void) {
         _cleanup_close_ int fd = -1;
         int r;
-        bool writable;
+        bool writable = false;
         struct stat st;
         char id[34]; /* 32 + \n + \0 */