avoid exposing the path for libfrtbegin and libg2c via -L to the linker

since that will confuse the finding of the proper libstdc++ and other
g++ libraries (especially when g77 is 3.x.y and g++ is 4.x.y).


git-svn-id: http://root.cern.ch/svn/root/trunk@13797 27541ba8-7e3a-0410-8455-c3a389f83636