Unbreak math/parmetis everywhere except FreeBSD 8

There were two issues with the previous commit.

• LDFLAGS wsa missing -L argument for libc++
• The libc++.so file is not actually a library, it's a linker script. Specifying libc++.so in LIB_DEPENDS causes the port to fail when it tries to install libc++ twice since it fails to recognize libc++.so as a library.

This doesn't fix FreeBSD breakage because it seems that it's missing
the C99 math function symbols.