- Move project definition to the top and add VERSION
 - Remove obsolete variable PROJECT and use standard PROJECT_NAME
 - Require a more modern version of CMake
 - Use CMAKE_CXX_STANDARD instead of manually adding -std=c++11
 - Only include ExternalProject if required

Rules from:

https://community.kde.org/Policies/CMake_Coding_Style#Indentation

Since rest of the codebase doesn't use tabs, CMake files shouldn't
either.

CMake sets PIC automatically for shared libraries and modules.

This enables PROJECT_* variables.

Don't add headers as direct dependencies to targets. Compilers can
detect header dependencies and manage recompilation automatically.

Use list(APPEND ...) in favor of the less explicit set(var ${var} ...).

Downcase variable names to separate project local variables to CMake
vars.

Set configuration parameters before adding tacopie subdirectory to
force wanted behavior.

A common use case is to integrate libraries as git submodules and a
single project might have multiple modules each with test dependencies
to gtest. Usually the top level project configures gtest, propagating
the target to it's submodules. This is preferable to the case where
each submodule uses ExternalProject to build & install gtest in a
custom location with possibly arbitrary target names (googletest,
gtest, <foobar>_gtest, etc.) as this can easily lead to multiple gtest
builds.

As a solution check if gtest (which is defined by googletest's build
files) is an existing target to determine if project's own googletest
submodule should be added to the build. Either way the subsequent
add_subdirectory() tests will have a target "gtest" to link
against. Various INTERFACE flags will take care of the rest.

A further improvement is to add "${PROJECT_NAME}::" as a prefix to the
tests. This helps when cpp_redis is a part of a larger project
and (for any reason) developers want to run cpp_redis' tests only
using "ctest -R cpp_redis".

They make maintaining the file extremely frustrating as they break all
the time. AFAIK the style

if (<condition>)

endif (<condition>)

is rarely seen anymore.

When building the include directories should be absolute paths, but
obviously when installing / exporting the paths should be relative.

 - add SOVERSION and RESOURCE properties

 The former results in a symlink during installation phase (on
 platforms that support such things) and the latter makes it easier to
 install extra stuff in install(TARGETS).

 - add EXPORT target name
 - remove unnecessary and harmful install(DIRECTORY) calls

Support for EXPORT_TARGET_NAME and better install support all around.

Using tacopie from system libdir might be good feature to have, but
the current implementation is buggy. Add it properly later using
pkgconfig or CMake imports.

The call will automatically install the project configuration as an
importable file called lib/cpp_redis/cpp_redis.cmake. The file will
configure installed targets as imports so that they can be used like
regular CMake targets.

Allow the configuration to go slightly further with older versions.

Also move the cmake_minimum_required() at the top of the file as per
cmake official recommendations.

CMake's OSx tarball contents are different than on Linux. Handle those
in travis.

Travis build file was getting harder to read. Extract the same steps
into scripts. Most of the files need to be sourced as they
conditionally set environment variables.

Tar creates a directory called ${CMAKE_FILE}, just use that in ${CMAKE_INSTALL_DIR}.

After the last change CMAKE_INSTALL_DIR was defined in terms of
CMAKE_DOWNLOAD_DIR.