Fix: configure_fastr works when invoked with relative path

(cherry picked from commit 46c4d04)

Author: steve-s

com.oracle.truffle.r.native/run/configure_fastr

@@ -42,7 +42,7 @@ done
   ./configure --with-x=no --with-aqua=no --enable-memory-profiling FFLAGS=-O2 $@
   ed Makeconf < edMakeconf.etc
   export R_DEFAULT_PACKAGES=base
-  R_LIBS_USER=`"$( dirname "$source" )/../bin/R" --slave -e 'cat(path.expand(Sys.getenv("R_LIBS_USER")))'`
+  R_LIBS_USER=`../bin/R --slave -e 'cat(path.expand(Sys.getenv("R_LIBS_USER")))'`
   echo "Creating user specific library directory: $R_LIBS_USER"
   mkdir -p "$R_LIBS_USER"
   echo "DONE"