# search a list of libraries for a given (C linkage) _symbol. returns the
# required library in _variable, and defines HAVE_SYMBOL if it was found.
macro(search_libs _variable _symbol)
    message (STATUS "searching for ${_symbol}")

    try_compile(
        __search_libs_${_symbol}
        "${CMAKE_BINARY_DIR}/CMakeTmp"
        SOURCES "${CMAKE_SOURCE_DIR}/cmake/search_libs.cpp"
        COMPILE_DEFINITIONS "-DSYMBOL=${_symbol}")

    if (NOT __search_libs_${_symbol})
        foreach (lib ${ARGN})
            if (NOT __search_libs_${_symbol}_last)
                message (STATUS "searching for ${_symbol} in ${lib}")
                try_compile(
                    __search_libs_${_symbol}
                    "${CMAKE_BINARY_DIR}/CMakeTmp"
                    SOURCES "${CMAKE_SOURCE_DIR}/cmake/search_libs.cpp"
                    LINK_LIBRARIES "-l${lib}"
                    COMPILE_DEFINITIONS "-DSYMBOL=${_symbol}")
                set (__search_libs_${_symbol}_last ${lib})
            endif ()
        endforeach()
    endif()

    if (__search_libs_${_symbol})
        if (__search_libs_${_symbol}_last)
            message (STATUS "searching for ${_symbol} - found (in ${__search_libs_${_symbol}_last})")
        else ()
            message (STATUS "searching for ${_symbol} - found")
        endif ()
        set (${_variable} __search_libs_${_symbol}_last)
        string (TOUPPER ${_symbol} upper_symbol)
        set (HAVE_${upper_symbol} 1)
    else ()
        message (STATUS "searching for ${_symbol} - not found")
    endif ()
endmacro()