Travis cleanups: install `libgtest-dev`, remove unused apt sources

…unused apt sources

Add thread-safe logger to http server test

test-compilation-and-link.sh: Add -std=c++17 -Wall -Werror to compile and link DEP-8 test...

Upgrade to C++17 and use std::optional

More traces to Peer class, http server test and change id generation

Make Travis less verbose

Fixes #858

Replace `Pistache::Optional` with `std::optional`

So that Travis CI builds do not fail if systemctl returns a bad status
code

But preserving error logs

Update clang-format in Travis

AllowShortBlocksOnASingleLine: Empty was added in clang-format 10

Fix warning: backslash-newline at end of file

include/pistache/meson.build: Fixed missing meta.h...

Fix pkg-config usage with Meson in Readme

Add project status in the Readme

Closes #894

Reapply #890

See #890

Revert "Meson improvements"

Meson improvements

…an/control

The .pc file is now generated using Meson generator, so there's no need
anymore to store a .pc.in file in the project root and then configure it
manually. All dependencies and libraries are automatically set based on
what the build target actually uses, so there's no need to manually
track all the required libs and flags. As a result, code is much cleaner

Previously there was a check that made it possible for the users to
switch between building a static or shared library when using Pistache
as a Meson subproject. Now Meson checks for the desired library type
before defining any build targets, and if somebody wants to only use the
static library it doesn't have to also build the shared one.

Also, code is now way cleaner, since the two calls to static_library and
shared_library are now a single library() call

Move pkg-config private libraries to Libs.private

#842: Add more debug traces to http server test