Change from spdlog_ex to const spdlog_ex& got rid of the GCC8 warning.

Added:
* Release/Debug job for gcc 7 and clang 3.5
* Debug asan gcc 7 job
* Debug tsan gcc 7 job
* Disabled extensions
* Added a spdlog::spdlog alias for tests

Removed:
* Valgrind workaround, the current version is 3.10.1
* install_libcxx
* Makefiles

Fixed:
* examples build
* multisink build

Workarounds:
* gcc7 with tsan and asan needs gold linker, otherwise build
  fails becase of unrecognized option '--push-state'

CMake 3.1 is used in main CMakeLists file, no reason not to use it in example.
Threads::Threads is introduced in cmake 3.1.

Marking project as CXX will disable detecting C compiler and other checks.
Removed 'INCLUDES DESTINATION' as it made the include path appear doubled.

It's no longer a standalone cmake file because cmake was crashing when
reading it as part of the subproject.