[patch] use -Wl,-rpath on freebsd in preference to -rpath

[John Hein]

-rpath may be accepted by clang & gcc on freebsd, but it's not
documented (by either compiler's documentation) and generally
-Wl,-rpath is preferred and has been supported for a long time (at
least as far back as gcc 2.95 in the base freebsd 4 tree).

See also discussion here:

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=178732

--- ncurses-6.0/aclocal.m4.orig 2015-08-06 00:46:34.000000000 +0000
+++ ncurses-6.0/aclocal.m4      2016-02-14 18:46:30.019293000 +0000
@@ -3134,13 +3134,13 @@ case $cf_cv_system_name in
                LD_RPATH_OPT="-rpath "
        fi
        ;;
-(linux*|gnu*|k*bsd*-gnu)
+(linux*|gnu*|k*bsd*-gnu|freebsd*)
        LD_RPATH_OPT="-Wl,-rpath,"
        ;;
 (openbsd[[2-9]].*|mirbsd*)
        LD_RPATH_OPT="-Wl,-rpath,"
        ;;
-(dragonfly*|freebsd*)
+(dragonfly*)
        LD_RPATH_OPT="-rpath "
        ;;
 (netbsd*)