maint: Clean up various usages of #ifdef. * randmtzig.c: Use #ifdef rather than just #if. * EditControl.h : Use '#if ! defined' rather than '#ifndef' in guard block to match Octave style. * dialog.cc, settings-dialog.cc: Add FIXME notes about questionable use of #if mechanism. * file-editor-tab.cc, Array-b.cc, Array-ch.cc, Array-d.cc, Array-f.cc, Array-i.cc, Array-s.cc, MatrixType.h, quit.h, lo-sysdep.cc, lo-cutils.c, oct-alloc.h, oct-shlib.cc, sparse-sort.h: #define HAVE_XXX macros to 1, not just empty but defined. * octave-txt-lexer.h, octave-cmd.h, octave-preserve-stream-state.h, txt-eng.h, zfstream.h, oct-conf-features.h, oct-conf.h: Use octave_ namespace prefix on name of #define used to prevent multiple inclusion of headers. * parser.h, webinfo.h, ov-oncleanup.h, op-int.h, display-available.h, shared-fcns.h: Add #define guard to prevent multiple inclusion. * quadcc.cc: use all capitals for #define MIN_CQUAD_HEAPSIZE. * ov-intx.h: Add note that this file must not use guard #define. * eigs-base.h, randmtzig.h: Write '! defined' rather than '!defined'. * file-ops.cc, oct-sparse.h: Use parentheses around complex #if tests. * oct-syscalls.cc, oct-base64.cc, statdefs.h: Indent #ifdef blocks correctly. * oct-conf-post.in.h: Use "! defined". Define macros to 1, not just empty but defined.