(CMAKE ROSE) re-enable building on Linux

* On the support versions of GCC, std::set::erase() returns void,
  not an iterator.  Use ifdefs within PtrSet.h to handle this
  difference in functionality.

* Fix a typo in src/CMakeLists.txt.  HAVE_SYS_MMAN_H is the name
  of the variable that determines whether or not we should attempt
  to compile MemoryMap.C, not HAVE_SYS_MANN_H.

* remove erroneous "return" within AstInterface.C

* resolve ambiguous conversion error within LoopTransformInterface.h