[Bug c++/61412] Warnings incorrectly suppressed when compiling previously preprocessed file

[matt at godbolt dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61412

--- Comment #3 from Matt Godbolt <matt at godbolt dot org> ---
Many apologies: this does indeed also happen with GCC 4.7.2! Thanks for
checking.

Also, re: the -Wsystem-headers, this does indeed fix this case but of course in
general isn't a good workaround as system headers often do have warnings.