Add suppressions for a few warnings that cannot be meaningfully fixed by
MariaDB developers.

Changes for XtraDB, PBXT, and YaSSL also submitted upstream.

Also add a `ccfilter` wrapper that can be used to filter out suppressed warnings in a
local build (to check that new warnings are not introduced).


client/mysqlbinlog.cc:
  Fix compiler warnings.
config/ac-macros/misc.m4:
  Fix wrong naming, autoconfig requires _cv_ in cached names.
extra/yassl/include/yassl_int.hpp:
  Fix compiler warnings.
extra/yassl/src/handshake.cpp:
  Fix compiler warnings.
extra/yassl/src/yassl_imp.cpp:
  Fix compiler warnings.
extra/yassl/src/yassl_int.cpp:
  Fix compiler warnings.
extra/yassl/taocrypt/include/modes.hpp:
  Fix compiler warnings.
extra/yassl/taocrypt/src/asn.cpp:
  Fix compiler warnings.
mysys/my_compress.c:
  Fix compiler warnings.
sql/mysqld.cc:
  Fix compiler warnings.
sql/strfunc.cc:
  Fix compiler warnings.
storage/pbxt/src/discover_xt.cc:
  Fix compiler warnings.
storage/xtradb/fil/fil0fil.c:
  Fix compiler warnings.
storage/xtradb/mtr/mtr0mtr.c:
  Fix compiler warnings.
storage/xtradb/srv/srv0srv.c:
  Fix compiler warnings.
storage/xtradb/srv/srv0start.c:
  Fix compiler warnings.
strings/decimal.c:
  Fix compiler warnings.
support-files/ccfilter:
  Add helper for suppressing compiler warnings in local developer source tree.
      
  Allows to check for not introducing new warnings into Buildbot without having to actually
  run the build through Buildbot.
support-files/compiler_warnings.supp:
  Suppress a few warnings that cannot be meaningfully fixed in source code.