portsで作ったgcc47だと
To ensure binaries built with this toolchain find appropriate versions
of the necessary run-time libraries, you may want to link using

-Wl,-rpath=/usr/local/lib/gcc47

For ports leveraging USE_GCC, USES=compiler, or USES=fortran this happens
transparently.
とか言われるよ
gccのlibを先に読ませる必要あるみたい