meson: re-enable -Wmaybe-uninitialized on -O[02] with recent gcc versions

The warning was disabled in 8794164fed to avoid
false positives. But it is useful in finding errors, even if it sometimes
results in untrue warnings (c.f. 77fac974fe, da46a1bc3c).

After #19168, #19169, and #19175, there are no warnings with
-Dbuildtype=debug-optimized/-O2 and gcc-11.0.1-0.3.fc34.x86_64. Warnings
are reenabled for -O[23]

-O0 is good for development, and -O2 is the default optimization level for
Fedora package builds. -Os, -O3, -O1, and -Og still generate some warnings. In
fact, with -Os the number of warnings seems completely hopeless. Dozens and
dozens.

meson.build

@@ -395,10 +395,6 @@ possible_cc_flags = [
         '-Wno-error=#warnings',  # clang
         '-Wno-string-plus-int',  # clang
 
-        # Disable -Wmaybe-uninitialized, since it's noisy on gcc 8 with
-        # optimizations enabled, producing essentially false positives.
-        '-Wno-maybe-uninitialized',
-
         '-ffast-math',
         '-fno-common',
         '-fdiagnostics-show-option',
@@ -409,6 +405,15 @@ possible_cc_flags = [
         '--param=ssp-buffer-size=4',
 ]
 
+# Disable -Wmaybe-unitialized when compiling with -Os/-O1/-O3/etc. There are
+# too many false positives with gcc >= 8. Effectively, we only test with -O0
+# and -O2; this should be enough to catch most important cases without too much
+# busywork. See https://github.com/systemd/systemd/pull/19226.
+if cc.get_id() == 'gcc' and (not '02'.contains(get_option('optimization')) or
+                             cc.version().version_compare('<10'))
+        possible_cc_flags += '-Wno-maybe-uninitialized'
+endif
+
 # --as-needed and --no-undefined are provided by meson by default,
 # run mesonconf to see what is enabled
 possible_link_flags = [