Based on work by Stan.
I have removed the HAVE_ICONV_CONST and ICONV_CONST=const flags which are unneeded, replacing them with LIBICONV_PLUG, which instructs FreeBSD iconv implementation to act as a plug in replacement for the gnu standalone implementation.
The -rdynamic flag is removed since it causes warnings.
I observed nvtt uses the wrong include for sse2, so I replaced it.
Please note that to make nvtt compile on FreeBSD on i386 (32 bit hardware), the -msse2 option is required to clang to enable the CPU features required by nvtt. I've been doing thiss via the ports framework.