Add checks for C++11 compilers.
Enable C++11 compiler options.
Remove equal_strings function.

This is like an unoptimized strcmp.
Fix #include guard typo.
Fix build errors in optional targets.
Use nullptr.
Remove deprecated register storage class.
Remove unused recreate_command_line function.
Fix incorrect deallocator.
Fix possible leaks.
Use range-based loops.

Mostly using 'auto', unless the loop specifically had a type in use
somewhere.
Convert DIR_ENTRY into a shared_ptr.
Rename Clear as reset in REFptr.

This makes it easier to incrementally change from REFptr to the standard
shared_ptr instead.
Make AUX_JOB use a shared_ptr.
Make BaseStrokeOffsetLIST use a shared_ptr.
Make CLEARobs use a shared_ptr.
Make VIEW_EASEL use a shared_ptr.
Make TEXTURE use a shared_ptr.
Use dynamic_cast instead of custom upcast function.

Unfortunately, this is not always possible, since REFptrs hide the real
pointer, but once they're replaced, this can be done more consistently.
Fix missing semi-colon.