configure: add --enable-rpath

This option facilitates testing shared libarary builds: for instance
fate builders do no longer need to set LD_LIBRARY_PATH as the binaries will
get the right search paths hardcoded into their executable file.

This option is only meant to be used for testing purposes: The installed
libraries must not move around in the file system, and doing so will
cause a lot of subtle problems. For more information why using RPATH is
dangerous, please refer to

https://blog.flameeyes.eu/2010/06/the-why-and-how-of-rpath

configure

@@ -86,6 +86,7 @@ Standard options:
   --shlibdir=DIR           install shared libs in DIR [PREFIX/lib]
   --incdir=DIR             install includes in DIR [PREFIX/include]
   --mandir=DIR             install man page in DIR [PREFIX/share/man]
+  --enable-rpath           use rpath when linking programs [USE WITH CARE]
 
 Licensing options:
   --enable-gpl             allow use of GPL code, the resulting libs
@@ -1596,6 +1597,7 @@ CMDLINE_SELECT="
     logging
     lto
     optimizations
+    rpath
 "
 
 PATHS_LIST="
@@ -4293,6 +4295,7 @@ check_disable_warning -Wno-pointer-sign
 # add some linker flags
 check_ldflags -Wl,--warn-common
 check_ldflags -Wl,-rpath-link=libswscale:libavfilter:libavdevice:libavformat:libavcodec:libavutil:libavresample
+enabled rpath && add_ldexeflags -Wl,-rpath,$libdir
 test_ldflags -Wl,-Bsymbolic && append SHFLAGS -Wl,-Bsymbolic
 
 # add some strip flags