CMake: find brew-installed openssl on Mac

FindBrewOpenSSL.cmake

macro(find_openssl_libraries)
    if (APPLE)
        # If we're on OS X check for Homebrew's copy of OpenSSL instead of Apple's
        if (NOT OpenSSL_DIR)
            find_program(HOMEBREW brew)
            if (HOMEBREW STREQUAL "HOMEBREW-NOTFOUND")
                message(WARNING "Homebrew not found: not using Homebrew's OpenSSL")
                if (NOT OPENSSL_ROOT_DIR)
                    message(WARNING "Use -DOPENSSL_ROOT_DIR for non-Apple OpenSSL")
                endif()
            else()
                execute_process(COMMAND brew --prefix openssl
                    OUTPUT_VARIABLE OPENSSL_ROOT_DIR
                    OUTPUT_STRIP_TRAILING_WHITESPACE)
            endif()
        endif()
    endif()
    find_package(OpenSSL ${ARGN})
    if (OPENSSL_FOUND)
        include_directories(SYSTEM ${OPENSSL_INCLUDE_DIR})
    endif()
endmacro(find_openssl_libraries)