patch 8.1.2253: using "which" to check for an executable is not reliable

Problem:    Using "which" to check for an executable is not reliable.
Solution:   Use "command -v" instead.  Also exit with error code when
            generating tags has an error. (closes #5174)
diff --git a/runtime/doc/Makefile b/runtime/doc/Makefile
index 4ca0881..e10415f 100644
--- a/runtime/doc/Makefile
+++ b/runtime/doc/Makefile
@@ -323,9 +323,9 @@
 # Use Vim to generate the tags file.  Can only be used when Vim has been
 # compiled and installed.  Supports multiple languages.
 vimtags: $(DOCS)
-	@if which $(VIMEXE) >/dev/null; then \
-	  $(VIMEXE) --clean -eX -u doctags.vim >/dev/null; \
-	  echo "help tags updated"; \
+	@if command -v $(VIMEXE); then \
+	    $(VIMEXE) --clean -eX -u doctags.vim >/dev/null && \
+	      echo "help tags updated"; \
 	  else echo "vim executable $(VIMEXE) not found; help tags not updated"; fi
 
 # Use "doctags" to generate the tags file.  Only works for English!
diff --git a/src/version.c b/src/version.c
index 9b4c386..2a09818 100644
--- a/src/version.c
+++ b/src/version.c
@@ -742,6 +742,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2253,
+/**/
     2252,
 /**/
     2251,