Summary:
Primarily, this change adds a way to work around a bug limiting the effective output (and therefore debugability) of the Linux builds using parallel make. We would get
make[1]: write error: stdout
probably due to a kernel bug, apparently affecting both available ubuntu 16 machine images (maybe not affecting docker images, less horsepower). https://bugs.launchpad.net/ubuntu/+source/linux-signed/+bug/1814393

Now in the CircleCI config, make output on Ubuntu is piped through a custom 'cat' that ignores EAGAIN errors, which seems to fix the problem.

Significant other changes:
* Add another linux build that combines
  * LIB_MODE=shared, to ensure this works with compile and unit test execution
  * Alternative rocksdb namespace, to ensure this works (not rely on Travis)
  * ASSERT_STATUS_CHECKED=1, but with building all unit tests and running those expected to pass with it
* Run release build with and without gflags. (Was running only without, ignore large swaths of code in a normal release build! Two regressions in this build, only with gflags, in the last week not caught by CI!)
* Use gflags with unity and LITE build, as typical case.

Debugability improvements:
* Use V=1 to show commands being executed (thanks to EAGAIN work-around)
* Print kernel version and compiler versions as part of V=1 output from Makefile

Cosmetic other changes:
* Put more commands on one line, for less clutter in CircleCI output pages
* Remove redundant "all" in "make all check" and put make command options before targets
* Change some recursive "make clean" into dependency on "clean," toward minimizing unnecessary overhead (detect platform, build version, etc.) of extra recursive makes

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7078

Reviewed By: siying

Differential Revision: D22391647

Pulled By: pdillinger

fbshipit-source-id: d446fccf5a8c568b37dc8748621c8a5c546fe135