Fix warnings with Cerbero's mingw (gcc 4.7)

orcutils.h:59:38: error: "__STDC_VERSION__" is not defined [-Werror=undef]
1 job for fix-stdc-version-undefined-error in 1 minute and 57 seconds (queued for 7 minutes and 28 seconds)
Status Job ID Name Coverage
  Build
passed #99558
debian sid

00:01:57