Superblocks rebased for merge

Method to get dictionary header size

Specs. clarifications for Block_Maximum_Size

as a follow up of #1882

Change c90 to c89

Fix parameter selection and adjustment with srcSize == 0

This reverts commit a152b4c67a5266f611db4a2eac4a79003852a795.

c89 and c90 are the same language. Old compilers like gcc 4.2.1 don't
know the c90 alias.

check that the dictBuilder can be compiled
with only lib/common and lib/compress as dependencies.

Fix #1861: Restrict overlapLog Parameter When Not Built With Multithreading

Take ZSTD_parameters as a const pointer

'head -c BYTES' is non-portable.

'date -r FILE' is non-portable.

This parameter is unused in single-threaded compression. We should make it
behave like the other multithread-only parameters, for which we only accept
zero when we are not built with multithreading.

date(1) is used to display the last modification time of a file, which
is not supported on OpenBSD, FreeBSD and Darwin. Instead use stat(1).

Tested on OpenBSD.

Similar to #1321: The '-c' option for 'head' is non-portable (not in
POSIX).

[clean up] Replacing some literals with constants in zstd_compress_literals.c