ports/math/gsl and ports/math/fftw don't use libtool from ports.
It can cause problems with multithreaded programs that uses
this libraries. Such program will be linked with both libc.so and
Using libtool from ports fixes the problem.
I think port maintainers must correct this ports.
How-To-Repeat: Just build this ports and check is dependencies:
[/usr/ports/math/fftw]# ldd /usr/local/lib/libfftw.so
libm.so.2 => /usr/lib/libm.so.2 (0x2812d000)
libc.so.4 => /usr/lib/libc.so.4 (0x28069000)
Over to Maintainer of fftw port
Fix libtool usage and check that libc dependency of shared libs are avoided.