config.h is generated by CMake's configure_file(), which is normally
smart enough to not update the file's timestamp if the file is
unchanged. However, config.h.in #defines LCB_BUILD_TIMESTAMP which will
be different every time CMake is run. This means config.h will be
updated every time, and since most .cc files directly or indirectly
include config.h, this causes almost everything to be recompiled.

This change introduces a separate build-timestamp.h.in, which contains
only #define LCB_BUILD_TIMESTAMP. This in turn is only included by a new
file cbc-timestamp.cc, which defines a const global variable
lcb_build_timestamp with that value. Thus, only this tiny file will get
rebuilt every time CMake is run.

Change-Id: I67c78fed4c33be911427dfe1dfd97a258eb2e197
Reviewed-on: https://review.couchbase.org/c/libcouchbase/+/201943
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>

Change-Id: I84f22435403c7dce187e31e83372d299a949667e
Reviewed-on: https://review.couchbase.org/c/libcouchbase/+/202863
Reviewed-by: <ankit.prabhu@couchbase.com>
Reviewed-by: Rishit Chaudhary <rishit.chaudhary@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>

Change-Id: Ifb8507fd3a925e645fa8da9d1a2ace2a7c815f59
Reviewed-on: https://review.couchbase.org/c/libcouchbase/+/202982
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>