Improved rootsys.m4

Replaced AC_CHECK_PROGS by AC_PATH_PROG. Using absolute paths to tools
in Makefiles is preferable.
Moved CINT pragmas to LinkDef header

* Having the CINT pragmas in the class header files makes external code
  that includes the headers generate dictionaries again.
Changed Iterator::min() and max() to return NaN if empty.
Changed FindMax and FindMin to throw exception if input is empty
Added missing return statements to some methods of WlDecomp classes
Added missing include in util.h
Improved WlDecomp state handling and added copying with other coeff storage
Changed parameter "to" of getCoeffLocation in WlDecomp "until"

New semantics: until = to + stride
Added method WlDecomp1X::resizeDecomp
Made build compatible with ROOT-6