Commit 3237ce1557467a3f89a3d1fa7a9308161665bf0c fixed the build on CURRENT with a heavy hammer that made register a macro. This change passes -Wno-register to the compiler instead, which is cleaner.
For the record: the current change restricts the change to __FreeBSD_version while this one checks for a compiler version instead -- the former could work if one's building the ports tree with GCC even though the FreeBSD release matters less than the GCC version.