build: Don't use any -Werror in release builds

Do kep them for debug and debugoptimized builds though.

Keeping -Werror flags in release builds causes issues with forward
compatibility, when new compiler releases or different toolchains
suddenly cause those warnings to be emitted during compilation.

While we certainly want those issues to be investigated and fixed, they
should not prevent anyone from building GTK until they are.

Resolves #4388

meson.build

@@ -291,6 +291,9 @@ elif cc.get_id() == 'gcc' or cc.get_id() == 'clang'
     '-Wundef',
     '-Wuninitialized',
     '-Wunused',
+  ]
+  
+  extra_warnings = [
     '-Werror=address',
     '-Werror=array-bounds',
     '-Werror=empty-body',
@@ -312,6 +315,16 @@ elif cc.get_id() == 'gcc' or cc.get_id() == 'clang'
     '-Werror=write-strings',
   ]
 
+  if get_option('buildtype').startswith('debug')
+    foreach warning: extra_warnings
+      test_cflags += '-Werror=@0@'.format(warning)
+    endforeach
+  else
+    foreach warning: extra_warnings
+      test_cflags += '-W@0@'.format(warning)
+    endforeach
+  endif
+
   if cc.get_id() == 'gcc'
     test_cflags += ['-Wcast-align'] # This warns too much on clang
   endif