meson: make sure preprocesor warnings are not treated as errors

Clang includes -W#warning in -Werror, so the #warning used for msan would
be an error.

v2:
- use -Wno-error=... so that the warning is still emitted, but not as an error.

meson.build

@@ -333,6 +333,7 @@ possible_cc_flags = [
         '-Wno-missing-field-initializers',
         '-Wno-unused-result',
         '-Wno-format-signedness',
+        '-Wno-error=#warnings',
 
         # work-around for gcc 7.1 turning this on on its own.
         '-Wno-error=nonnull',