Made the library installable

[01e9]

Now the library can be installed as a cmake external project or directly in os.

Also it creates a cmake package which can be used like this:

find_package(restc-cpp REQUIRED)
target_link_libraries(${PROJECT_NAME} PRIVATE restc-cpp)

Test

Create a project with these 2 files:

CMakeLists.txt

cmake_minimum_required(VERSION 3.8)
project(restc_cpp_test)

set(EXTERNAL_PROJECTS_PREFIX ${CMAKE_BINARY_DIR}/external-projects)
set(EXTERNAL_PROJECTS_INSTALL_PREFIX ${EXTERNAL_PROJECTS_PREFIX}/installed)
include(ExternalProject)
ExternalProject_Add(externalRestcCpp
    PREFIX "${EXTERNAL_PROJECTS_PREFIX}"
    GIT_REPOSITORY "https://github.com/arteniioleg/restc-cpp.git"
    GIT_TAG "master"
    CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${EXTERNAL_PROJECTS_INSTALL_PREFIX}
    )
if(NOT EXISTS "${EXTERNAL_PROJECTS_INSTALL_PREFIX}/include")
    # Happens on first CMake run.
    # Can't continue because the below `find_package(REQUIRED)` will fail.
    # Build all external project targets then rerun CMake and build the project target.
    message(AUTHOR_WARNING "Build all external projects then reload cmake.")
    return()
endif()

add_executable(${PROJECT_NAME} main.cpp)
set_target_properties(${PROJECT_NAME} PROPERTIES
    CXX_STANDARD 14
    CXX_STANDARD_REQUIRED YES
    CXX_EXTENSIONS NO
    )

add_dependencies(${PROJECT_NAME} externalRestcCpp)
find_package(restc-cpp REQUIRED HINTS ${EXTERNAL_PROJECTS_INSTALL_PREFIX})
target_link_libraries(${PROJECT_NAME} PRIVATE restc-cpp)

main.cpp

#include <iostream>
#include <restc-cpp/restc-cpp.h>

int main()
{
    auto rest_client = restc_cpp::RestClient::Create();
    std::string body;

    rest_client->Process([&body](restc_cpp::Context& ctx){
        auto reply = ctx.Get("https://github.com/jgaa/restc-cpp/raw/master/.gitignore");
        body = reply->GetBodyAsString();
    });
    rest_client->CloseWhenReady(true);

    std::cout << "TEST OK" << std::endl << body << std::endl;

    return 0;
}

Build

1. mkdir build
2. cd build
3. cmake ..
4. cmake --build . (build all external projects)
5. cmake .. (this will pass return() and will add_executable())
6. cmake --build . --target restc_cpp_test
7. ./restc_cpp_test

[jgaa]

Thanks for the patch.
I will test it tomorrow on all the supported platforms, and update the documentation before I merge it to master.

Jarle