dnl @synopsis AX_ISC_RPATH
dnl
dnl @summary figure out whether and which "rpath" linker option is available
dnl
dnl This macro checks if the linker supports an option to embed a path
dnl to a runtime library (often installed in an uncommon place), such as the
dnl commonly used -R option.  If found, it sets the ISC_RPATH_FLAG variable to
dnl the found option flag.  The main configure.ac can use it as follows:
dnl if test "x$ISC_RPATH_FLAG" != "x"; then
dnl     LDFLAGS="$LDFLAGS ${ISC_RPATH_FLAG}/usr/local/lib/some_library"
dnl fi
dnl
dnl If you pass --disable-rpath to configure, ISC_RPATH_FLAG is not set

AC_DEFUN([AX_ISC_RPATH], [

AC_ARG_ENABLE(rpath,
    [AS_HELP_STRING([--disable-rpath],[don't hardcode library path into binaries])],
    rpath=$enableval, rpath=yes)

if test x$rpath != xno; then
    # We'll tweak both CXXFLAGS and CCFLAGS so this function will work
    # whichever language is used in the main script.  Note also that it's not
    #LDFLAGS; technically this is a linker flag, but we've noticed $LDFLAGS
    # can be placed where the compiler could interpret it as a compiler
    # option, leading to subtle failure mode.  So, in the check below using
    # the compiler flag is safer (in the actual Makefiles the flag should be
    # set in LDFLAGS).
    CXXFLAGS_SAVED="$CXXFLAGS"
    CXXFLAGS="$CXXFLAGS -Wl,-R/usr/lib"
    CCFLAGS_SAVED="$CCFLAGS"
    CCFLAGS="$CCFLAGS -Wl,-R/usr/lib"

    # check -Wl,-R and -R rather than gcc specific -rpath to be as portable
    # as possible.  -Wl,-R seems to be safer, so we try it first.  In some
    # cases -R is not actually recognized but AC_LINK_IFELSE doesn't fail due
    # to that.
    AC_MSG_CHECKING([whether -Wl,-R flag is available in linker])
    AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[]])],
        [AC_MSG_RESULT(yes)
            ISC_RPATH_FLAG=-Wl,-R
        ],
        [AC_MSG_RESULT(no)
            AC_MSG_CHECKING([whether -R flag is available in linker])

            # Apple clang 5.1 is now considers unknown parameters
            # passed to linker (ld) as errors.  However, the same
            # unknown parameters passed to compiler (g++ ) are merely
            # treated as warnings. To make sure that we pick those
            # up, is to use -Werror.
            CXXFLAGS="$CXXFLAGS_SAVED -R/usr/lib"
            CCFLAGS="$CCFLAGS_SAVED -R/usr/lib"
            AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[]])],
                [AC_MSG_RESULT([yes; note that -R is more sensitive about the position in option arguments])
                    ISC_RPATH_FLAG=-R],
                [AC_MSG_RESULT(no)])
        ]
    )

    CXXFLAGS=$CXXFLAGS_SAVED
    CCFLAGS=$CCFLAGS_SAVED
fi

])dnl AX_ISC_RPATH