patch 8.1.2128: renamed libvterm sources makes merging difficult

Problem:    Renamed libvterm sources makes merging difficult.
Solution:   Rename back to the original name and only rename the .o files.
            Also clean the libvterm build artifacts. (James McCoy,
            closes #5027)
diff --git a/src/Makefile b/src/Makefile
index f169548..c084463 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2860,6 +2860,7 @@
 	-rm -f runtime pixmaps
 	-rm -rf $(APPDIR)
 	-rm -rf mzscheme_base.c
+	-rm -rf libvterm/.libs libterm/t/.libs libvterm/src/*.o libvterm/src/*.lo libvterm/t/*.o libvterm/t/*.lo libvterm/t/harness libvterm/libvterm.la
 	if test -d $(PODIR); then \
 		cd $(PODIR); $(MAKE) prefix=$(DESTDIR)$(prefix) clean; \
 	fi
@@ -3453,36 +3454,39 @@
 Makefile:
 	@echo The name of the makefile MUST be "Makefile" (with capital M)!!!!
 
+# Build rules for libvterm.  Putting them here allows for adding compilation
+# options specific for Vim.  Since the .o files go into objects/ we do need to
+# prefix vterm_ to avoid name clashes.
 CCCTERM = $(CCC_NF) $(VTERM_CFLAGS) $(ALL_CFLAGS) -DINLINE="" \
 	  -DVSNPRINTF=vim_vsnprintf \
 	  -DIS_COMBINING_FUNCTION=utf_iscomposing_uint \
 	  -DWCWIDTH_FUNCTION=utf_uint2cells
 
-objects/encoding.o: libvterm/src/encoding.c $(TERM_DEPS)
+objects/vterm_encoding.o: libvterm/src/encoding.c $(TERM_DEPS)
 	$(CCCTERM) -o $@ libvterm/src/encoding.c
 
-objects/keyboard.o: libvterm/src/keyboard.c $(TERM_DEPS)
+objects/vterm_keyboard.o: libvterm/src/keyboard.c $(TERM_DEPS)
 	$(CCCTERM) -o $@ libvterm/src/keyboard.c
 
-objects/termmouse.o: libvterm/src/termmouse.c $(TERM_DEPS)
-	$(CCCTERM) -o $@ libvterm/src/termmouse.c
+objects/vterm_mouse.o: libvterm/src/mouse.c $(TERM_DEPS)
+	$(CCCTERM) -o $@ libvterm/src/mouse.c
 
-objects/parser.o: libvterm/src/parser.c $(TERM_DEPS)
+objects/vterm_parser.o: libvterm/src/parser.c $(TERM_DEPS)
 	$(CCCTERM) -o $@ libvterm/src/parser.c
 
-objects/pen.o: libvterm/src/pen.c $(TERM_DEPS)
+objects/vterm_pen.o: libvterm/src/pen.c $(TERM_DEPS)
 	$(CCCTERM) -o $@ libvterm/src/pen.c
 
-objects/termscreen.o: libvterm/src/termscreen.c $(TERM_DEPS)
-	$(CCCTERM) -o $@ libvterm/src/termscreen.c
+objects/vterm_screen.o: libvterm/src/screen.c $(TERM_DEPS)
+	$(CCCTERM) -o $@ libvterm/src/screen.c
 
-objects/state.o: libvterm/src/state.c $(TERM_DEPS)
+objects/vterm_state.o: libvterm/src/state.c $(TERM_DEPS)
 	$(CCCTERM) -o $@ libvterm/src/state.c
 
-objects/unicode.o: libvterm/src/unicode.c $(TERM_DEPS)
+objects/vterm_unicode.o: libvterm/src/unicode.c $(TERM_DEPS)
 	$(CCCTERM) -o $@ libvterm/src/unicode.c
 
-objects/vterm.o: libvterm/src/vterm.c $(TERM_DEPS)
+objects/vterm_vterm.o: libvterm/src/vterm.c $(TERM_DEPS)
 	$(CCCTERM) -o $@ libvterm/src/vterm.c
 
 CCCDIFF = $(CCC_NF) $(ALL_CFLAGS)
@@ -4111,27 +4115,27 @@
  proto/gui_beval.pro structs.h regexp.h gui.h alloc.h ex_cmds.h spell.h \
  proto.h globals.h
 objects/gui_gtk_gresources.o: auto/gui_gtk_gresources.c
-objects/encoding.o: libvterm/src/encoding.c libvterm/src/vterm_internal.h \
+objects/vterm_encoding.o: libvterm/src/encoding.c libvterm/src/vterm_internal.h \
  libvterm/include/vterm.h libvterm/include/vterm_keycodes.h \
  libvterm/src/encoding/DECdrawing.inc libvterm/src/encoding/uk.inc
-objects/keyboard.o: libvterm/src/keyboard.c libvterm/src/vterm_internal.h \
+objects/vterm_keyboard.o: libvterm/src/keyboard.c libvterm/src/vterm_internal.h \
  libvterm/include/vterm.h libvterm/include/vterm_keycodes.h \
  libvterm/src/utf8.h
-objects/termmouse.o: libvterm/src/termmouse.c libvterm/src/vterm_internal.h \
+objects/vterm_mouse.o: libvterm/src/mouse.c libvterm/src/vterm_internal.h \
  libvterm/include/vterm.h libvterm/include/vterm_keycodes.h \
  libvterm/src/utf8.h
-objects/parser.o: libvterm/src/parser.c libvterm/src/vterm_internal.h \
+objects/vterm_parser.o: libvterm/src/parser.c libvterm/src/vterm_internal.h \
  libvterm/include/vterm.h libvterm/include/vterm_keycodes.h
-objects/pen.o: libvterm/src/pen.c libvterm/src/vterm_internal.h \
+objects/vterm_pen.o: libvterm/src/pen.c libvterm/src/vterm_internal.h \
  libvterm/include/vterm.h libvterm/include/vterm_keycodes.h
-objects/state.o: libvterm/src/state.c libvterm/src/vterm_internal.h \
+objects/vterm_state.o: libvterm/src/state.c libvterm/src/vterm_internal.h \
  libvterm/include/vterm.h libvterm/include/vterm_keycodes.h
-objects/termscreen.o: libvterm/src/termscreen.c libvterm/src/vterm_internal.h \
+objects/vterm_screen.o: libvterm/src/screen.c libvterm/src/vterm_internal.h \
  libvterm/include/vterm.h libvterm/include/vterm_keycodes.h \
  libvterm/src/rect.h libvterm/src/utf8.h
-objects/unicode.o: libvterm/src/unicode.c libvterm/src/vterm_internal.h \
+objects/vterm_unicode.o: libvterm/src/unicode.c libvterm/src/vterm_internal.h \
  libvterm/include/vterm.h libvterm/include/vterm_keycodes.h
-objects/vterm.o: libvterm/src/vterm.c libvterm/src/vterm_internal.h \
+objects/vterm_vterm.o: libvterm/src/vterm.c libvterm/src/vterm_internal.h \
  libvterm/include/vterm.h libvterm/include/vterm_keycodes.h \
  libvterm/src/utf8.h
 objects/xdiffi.o: xdiff/xdiffi.c xdiff/xinclude.h auto/config.h \