use like this:

    flags {
        linker("-lbla")
    }

works with conditionals:

    flags if #(os::_WIN32){
        linker("-lW32_bla")
    }

eventually macro support will allow generating these from configure tests,
something like:

    flags {
      @pkgconfig("gtk")
    }

in order to integrate into C conditionals they are emitted as pragma.
but there's really only msvc that supports linker pragmas,
so make also parses them back out of the c file and appends them to the
compiler commandline.

this means export to other build systems becomes slightly inconsistent,
but it would have been that way anyway, because every build system works
differently.

- make: we can just prepend a -E phase to each compile unit,
- python and nodejs: same as script.
- rust: same as buildscript
- golang: would have never worked anyway. best shot is emit each variant
  as different golang file?
- cmake: no idea. cmake is complicated

using c conditionals is more consistent,
and the variants never really worked anyway across library boundaries