add support for out-of-source building of binaries. Normal build is unaffected.

To make an out of source build do, assuming the source is in ~/root:
   mkdir ~/root-x8664
   cd ~/root-x8664
   ~/root/configure
   make
This is convenient to build e.g. 32 and 64-bit version from one source, but
also needed for cross-compilation, where it is now possible to build in
a special directory only the compile time tools, like rootcint for the
host architecture while the binary is build for the remote architecture
(like iOS).


git-svn-id: http://root.cern.ch/svn/root/trunk@36659 27541ba8-7e3a-0410-8455-c3a389f83636