* Set default build type using _INIT variable

Language support checking offers a standardized method to set default
build type using CMAKE_BUILD_TYPE_INIT variable. The variable must be
set before enable_language is called implicitly from project

* Enable only C++ compiler

* Let C++ compiler detection to check for standard support

Setting CMAKE_CXX_STANDARD variables before implicit call to
enable_language handles checking for standard support automatically.

* Fix pistache_shared export

Exporting targets allows find_package(Pistache) to import them
correctly. The import makes it easier to use Pistache in a porject using
cmake build system.

* Add wildcard ignore for multiple build directories

I used multiple build directories to test different configurations work.
My alternative test configuration is a Debian Stretch chroot for an old
distro test.

* Detect Threads which is implicit requirement

Static library requires PistancheConfig.cmake to detect all dependencies
to allow simple linking to imported library to work.

* Use find_package to detect OpenSSL

Add OpenSSL configure time detection for the development package.
FindOpenSSL.cmake started support IMPORTED libraries only in 3.4. The
IMPORTED library helps users to link correctly to pistache static
library.

Headers use libssl directly which forces pistache to use public
definition and library link.

* Export include directories to IMPORTED library

* Do not add pistache to global build configuration

PistacheConfig shouldn't add include directories and libraries to the
global build configuration. Imported libraries provide all required
build configurations to a depending target.

* Check if atomic library is required

Pistache should link to atomic library only if system requires the
library. The easiest method appears to be use a modified LLVM
CheckAtomic helper.

* Use target include directories for GTest

* Detect libcurl and remove pthread linking

* Update CMake Pistache example

* Add CMake ConfigVersion file

* Update the date part of version