Turn off format warnings in appveyor for mingw-w64

It appears -Wformat is broken in mingw-w64's gcc at present.
For example, see discussion at:
https://github.com/facebook/rocksdb/pull/2052#discussion_r108785499

As there is no easy fix, turn off format warnings in appveyor
configuration to reduce clutter in the output.

.appveyor.yml

@@ -30,7 +30,7 @@ build_script:
   - set HOME=.
   - set MSYSTEM=MINGW64
   - set PATH=C:/msys64/usr/bin;C:/msys64/mingw64/bin;%PATH%
-  - "sh -lc \"aclocal && autoheader && autoconf && ./configure && make -j2\""
+  - "sh -lc \"aclocal && autoheader && autoconf && ./configure CFLAGS='-Wno-format -g -O2' && make -j2\""
 
 #build_script:
 #  - make