[tex-live] building from source

[Karl Berry]

    > Ok, so the question is why and how is __USE_GNU being defined.
    > It's not explicitly in the TeX sources anywhere.

    The gnu-cpp docs hint at using __GNUC__ instead.

__GNUC__ has a different purpose (saying what compiler is in use).

Linux stdio.h explicitly does an #ifdef on __USE_GNU for declaring
getline.  Therefore that specific symbol has to be getting defined
somewhere on Cygwin, and the commands I sent should say where ...