Fix www/lynx cross-build by setting BUILD_CC.
Kludgily fix tcsh cross-build by passing NATIVE_CC.

Should patch the configure script to respect the environment setting
of CC_FOR_GETHOST (or CC_FOR_BUILD or something), but this is less
work and will do for now.
Oops -- use the proper conditional for USE_CROSS_COMPILE.

Had been flailing when the CONFIGURE_ENV setting didn't take, but
this turned out to be a red herring, and I forgot to back it out
before commit.
Add a little list of things to do to improve cross-compilation.
Fix cross-build of security/nettle by setting CC_FOR_BUILD.