Respect CFLAGS/CXXFLAGS/LDFLAGS, fixes #66

Even in linking command CFLAGS should be respected, no matter
if all objects have already been compiled or not. Some flags
affect both linking and object compilation and users shouldn't
be required to figure them out. This is standard in most build
systems anyway.

Reorderd the linker line a bit, so it matches common practice,
although this doesn't matter technically (except that LDFLAGS
must go before libraries we link against).