meson: re-enable -Wunused-result with clang

We disabled it in f73fb7b742 in response to an
apparent gcc bug. It seems that depending on the combination of optimization
options, gcc still ignores (void). But this seems to work fine with clang, so
let's re-enable the warning conditionally.

meson.build

@@ -322,7 +322,6 @@ basic_disabled_warnings = [
         '-Wno-format-signedness',
         '-Wno-missing-field-initializers',
         '-Wno-unused-parameter',
-        '-Wno-unused-result',
 ]
 
 possible_common_cc_flags = [
@@ -366,6 +365,12 @@ if cc.get_id() == 'gcc' and (not '02'.contains(get_option('optimization')) or
         possible_common_cc_flags += '-Wno-maybe-uninitialized'
 endif
 
+# Disable -Wno-unused-result with gcc, see
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425.
+if cc.get_id() == 'gcc'
+        possible_common_cc_flags += '-Wno-unused-result'
+endif
+
 # --as-needed and --no-undefined are provided by meson by default,
 # run mesonconf to see what is enabled
 possible_link_flags = [