glibc: make compilation more pure

Remove datetime from nscd.
glibc: put back the nscd check, by $out instead of date

I don't know why they feel they need to check the compatibility by build date,
so I would keep check against $out, which is a better nix equivalent.

Also, expression refactoring (put comments out of hash-changing bash).
perl-5.16: make it binary deterministic
python: improve library determinism

1) Make the core python libraries deterministic.
2) Make the python libraries created by glib deterministic.

@vcunat put back the removal of share/gtk-doc in glib.
libgcrypt, libgpg-error, busybox: improve determinism
openssl: make it deterministic
groff, opensp: make deterministic
gcc-wrapper: make __DATE__/__TIME__ deterministic

...when NIX_ENFORCE_PURITY=1.

@vcunat corrected the date according to docs.
https://gcc.gnu.org/onlinedocs/cpp/Standard-Predefined-Macros.html
In order to handle the spaces well, the extraAfter array had to be
quoted more properly and appended by +=.