added new configure options:

  --with-cc
  --with-cxx
  --with-f77
  --with-ld
which allows the specification of alternative compilers, which, of course,
have to be compatible with the specified or auto detected platform.
This is ok:
  ./configure solarisgcc --with-cxx=g++-4.2
this not:
  ./configure macosxicc --with-cxx=g++-4.2 
Also root-config now supports the options --cc, --cxx, -f77 and --ld.
If no alternative compilers were specified these return a blank.
In test/Makefile.arch and example of root-config --cxx et al usage is
given, so that the alternative compilers specified in ./configure will
be used to build the test programs.


git-svn-id: http://root.cern.ch/svn/root/trunk@21342 27541ba8-7e3a-0410-8455-c3a389f83636