gcc 4.5 enables some warnings with -Wall that didn't even exist in previous
versions (so we can't just disable them generally). This adds a new autoconf
macro to check for compiler flags, and adds -Wno-switch and -Wno-enum-compare
to the compiler flags if they exist.
git-svn-id: https://warzone2100.svn.sourceforge.net/svnroot/warzone2100/trunk@10597 4a71c877-e1ca-e34f-864e-861f7616d084