build: Add more #ifdefs to build GUI when Qscintilla is not present. * libgui/src/m-editor/file-editor-interface.h, libgui/src/m-editor/file-editor-tab.h, libgui/src/m-editor/file-editor.h, libgui/src/m-editor/find-dialog.h, libgui/src/m-editor/lexer-octave-gui.h: Surround entire file with #ifdef HAVE_QSCINTILLA so that generated moc-*.cc file will not be present if Qscintilla is not installed. * libgui/src/main-window.cc: Add #ifdef HAVE_QSCINTILLA around instances of file_editor.