Add support for third party allocators. To build and link with, e.g. tcmalloc

do:
    ./configure --with-alloc-lib=libtcmalloc
or
    ./configure --with-alloc-libdir=/my/alloc/lib --with-alloc-lib=libtcmalloc
Note, as this only changes how root.exe (and apps using root-config --libs)
are linked, after configure you need to do:
    rm bin/root.exe
    make
to link with the new allocator.


git-svn-id: http://root.cern.ch/svn/root/trunk@34367 27541ba8-7e3a-0410-8455-c3a389f83636