Improve missing-program error handling in make_ctags and make_etags.

If ctags (resp. etags) isn't installed, these scripts naturally fail,
but the error messages were less clear than one could wish.
It seems worth installing an explicit test to improve that.

Nikolay Shaplov, with suggestions from Michael Paquier and Andrew Dunstan

Discussion: https://postgr.es/m/2394207.ccz7JgCJsh@x200m

src/tools/make_ctags

@@ -2,6 +2,9 @@
 
 # src/tools/make_ctags
 
+command -v ctags >/dev/null || \
+	{ echo "'ctags' program not found" 1>&2; exit 1; }
+
 trap "rm -f /tmp/$$" 0 1 2 3 15
 rm -f ./tags
 

src/tools/make_etags

@@ -2,6 +2,9 @@
 
 # src/tools/make_etags
 
+command -v etags >/dev/null || \
+	{ echo "'etags' program not found" 1>&2; exit 1; }
+
 rm -f ./TAGS
 
 find `pwd`/ -type f -name '*.[chyl]' -print |