The file config/mt-gnu currently defines CXXFLAGS_FOR_TARGET = $(CXXFLAGS) -D_GNU_SOURCE. To me, it seems that it should be += -D_GNU_SOURCE (like most of the other frags). Otherwise, I cannot override the CXXFLAGS without overriding CXXFLAGS_FOR_TARGET, etc.
-- Summary: config/mt-gnu CXXFLAGS_FOR_TARGET Product: gcc Version: 4.5.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: dougsemler at gmail dot com GCC build triplet: x86_64-unknown-linux-gnu GCC host triplet: x86_64-unknown-linux-gnu GCC target triplet: x86_64-unknown-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43538