build: use "-Wno-nonnull-compare" for building systemd

systemd uses that too. We cannot enable compiler warnings that
upstream doesn't want to support.

See-also: b59bce308d

configure.ac

@@ -1157,7 +1157,11 @@ fi
 
 NM_COMPILER_WARNINGS(AM_CFLAGS, ${more_warnings_default})
 
-NM_COMPILER_WARNING_FLAG(LIBSYSTEMD_NM_CFLAGS, "-Wno-gnu-variable-sized-type-not-at-end")
+for w in \
+    -Wno-nonnull-compare \
+    ; do
+    NM_COMPILER_WARNING_FLAG(LIBSYSTEMD_NM_CFLAGS, "$w")
+done
 AC_SUBST(LIBSYSTEMD_NM_CFLAGS)
 
 CC_CHECK_FLAGS_APPEND([with_cflags], [CFLAGS], [\

src/libnm-systemd-core/meson.build

@@ -28,6 +28,7 @@ libnm_systemd_core = static_library(
     top_inc,
     src_inc,
   ],
+  c_args: libnm_systemd_common_cflags,
   dependencies: [
     libnm_systemd_shared_dep_inc,
     glib_dep,

src/libnm-systemd-shared/meson.build

@@ -58,6 +58,7 @@ libnm_systemd_shared = static_library(
     top_inc,
     src_inc,
   ],
+  c_args: libnm_systemd_common_cflags,
   dependencies: glib_dep,
 )
 

src/meson.build

@@ -69,6 +69,14 @@ libn_dhcp4 = static_library(
 
 ###############################################################################
 
+libnm_systemd_common_cflags = [ ]
+
+libnm_systemd_common_cflags += cc.get_supported_arguments([
+  '-Wno-nonnull-compare',
+])
+
+###############################################################################
+
 subdir('libnm-std-aux')
 subdir('libnm-glib-aux')
 subdir('libnm-log-null')