* Support -Wsuggest-override

google/benchmark is C++11 compatible but doesn't use the `override` keyword.
Projects using google/benchmark with enabled `-Wsuggest-override` and `-Werror` will fail to compile.

* Add -Wsuggest-override cxx flag

* Revert unrelated formatting

* Revert unrelated formatting, take 2

* Revert unrelated formatting, take 3

* Disable -Wsuggest-override when compiling tests, gtest does not handle it yet
Co-authored-by: Dominic Hamon <dominichamon@users.noreply.github.com>

* Refactor BenchmarkInstance (precursor to #1105)

* fix bazel (debug) build

* clang-format on header

* fix build error on g++-4.8

* add g++-6 to ubuntu-14.04

* fix syntax

* fix yamllint errors for build-and-test

* fix 'add-apt-repository' command not found

* make 'run tests' explicit

* enable testing and run both release and debug

* oops

* Add ubuntu-14.04 build and test workflow

* avoid '.' in job name

* no need for fail fast

* fix workflow syntax

* install some stuff

* better compiler installations

* update before install

* just say yes

* trying to match up some paths

* Difference between runner and github context in container?

* Try some judicious logging

* cmake 3.5+ required

* specific compiler versions

* need git for googletest

* Disable testing on old compilers

* disable testing properly

This also fixes #1135. Because StrSplit was returning a vector with an
empty string, it was treated by PerfCounters::Create as a legitimate ask
for setting up a counter with that name. The empty vector is understood
by PerfCounters as "just return NoCounters()".

* Add API to benchmark allowing for custom context to be added

Fixes #525

* add docs

* Add context flag output to JSON reporter

* Plumb everything into the global context.

* Add googletests for custom context

* update docs with duplicate key behaviour

* Add `benchmark_context` flag that allows per-run custom context.

Add support for key-value flags in general.
Added test for key-value flags.
Added `benchmark_context` flag.
Output content of `benchmark_context` to base reporter.

Solves the first part of #525.

* Docs and better help

* Add 32-bit build support to build-and-test

* attempt different yaml multiline string format

* syntax fixes to yaml

* switch to getting alternative compilers working

* remove done TODO

* trying to separate out windows

* oops, typo.

* add TODOs for missing builds wrt travis

* Support optional, user-directed collection of performance counters

The patch allows an engineer wishing to drill into the root causes
of a regression, for example. Currently, only single threaded runs
are supported. The feature is a build-time opt in, and then a runtime
opt in.

The engineer may run the benchmark executable, passing a list of
performance counter names (using libpfm's naming scheme) at the
command line. The counter values will then be collected and reported
back as UserCounters.

This is different from #240 in that it is a benchmark user opt-in, and
the counter collection is transparent to the benchmark.

Currently, this is only supported on platforms where libpfm is
supported.

libpfm: http://perfmon2.sourceforge.net/

* 'Use' values param in Snapshot when BENCHMARK_OS_WINDOWS

This is to avoid unused parameter warning-as-error

* Added missing include for <vector> in perf_counters.cc

* Moved doc to docs

* Added license blurbs

Fixes #713

according to https://cmake.org/cmake/help/latest/command/function.html,
"Referencing to ARGV# arguments beyond ARGC have undefined behavior.",
which I hit with cmake 3.19.7.

This uses ARGC to check whether ARGV1 has been passed before referencing
it.

[tools] Fix dumb mistake in previous commit - print aggregates only means aggregates, not non-aggregates

This probably regressed in #1042.

Currently, i get:
```
Run on (32 X 7326.56 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x16)
  L1 Instruction 32 KiB (x16)
  L2 Unified 512 KiB (x16)
  L3 Unified 32768 KiB (x2)
```
which seems mostly right, except that the frequency is rather bogus.
Yes, i guess the CPU could theoretically achieve that,
but i have 3.6GHz configured, and scaling disabled.
So we clearly read the wrong thing.

With this fix, i now get the expected
```
Run on (32 X 3598.53 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x16)
  L1 Instruction 32 KiB (x16)
  L2 Unified 512 KiB (x16)
  L3 Unified 32768 KiB (x2)
```