apps: qcam: Fix clang build

While gcc ignores attempts to disable unknown warnings, clang does not
(`-Wunknown-warning-option`), thus compilation fails due to the recent
addition of `-Wno-sfinae-incomplete`. So only add it conditionally.

Closes: https://gitlab.freedesktop.org/camera/libcamera/-/issues/315
Fixes: aa2a0812e6 ("apps: qcam: Disable -Wsfinae-incomplete")
Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/apps/qcam/meson.build

@@ -40,7 +40,6 @@ qt6_cpp_args = [
     apps_cpp_args,
     '-DQT_NO_KEYWORDS',
     '-Wno-extra-semi',
-    '-Wno-sfinae-incomplete',
 ]
 
 # gcc 12 and 13 output a false positive variable shadowing warning with Qt
@@ -50,6 +49,10 @@ if qt6_dep.version().version_compare('>=6.9.0') and \
     qt6_cpp_args += ['-Wno-shadow']
 endif
 
+if cxx.get_id() == 'gcc' and cxx.version().version_compare('>=16')
+    qt6_cpp_args += ['-Wno-sfinae-incomplete']
+endif
+
 resources = qt6.preprocess(moc_headers : qcam_moc_headers,
                            qresources : qcam_resources,
                            dependencies : qt6_dep)