[tlbuild] popen/pclose declarations

[Karl Berry]

Hille,

Regarding the popen/pclose companion bug:

    [2] http://bugs.debian.org/64524

I rather suspect they are declared in the headers of all systems on
which one would compile TL nowadays, so my first thought is simply to
remove those declarations altogether from c-std.h.

If someone would like to try a compilation without them, that'd be
good, else I will in due course.

karl