Add watch target to Makefile for easier hacking

CONTRIBUTING.md

@@ -25,6 +25,22 @@ There is also a
 [stow-devel](https://lists.gnu.org/mailman/listinfo/stow-devel)
 mailing list (see [Mailing lists](README.md#mailing-lists)).
 
+Please be aware that all program source files (excluding the test
+suite) end in `.in`, and are pre-processed by `Makefile` into
+corresponding files with that prefix stripped before execution.  So if
+you want to test any modifications to the source, make sure that you
+change the `.in` files and then run `make` to regenerate the
+pre-processed versions before doing any testing.  To avoid forgetting
+(which can potentially waste a lot of time debugging the wrong code),
+you can automatically run `make` in an infinite loop every second via:
+
+    make watch
+
+(You could even use fancier approaches like
+[`inotifywait(1)`](https://www.man7.org/linux/man-pages/man1/inotifywait.1.html)
+or [Guard](https://guardgem.org/).  But those are probably overkill in
+this case where the simple `while` loop is plenty good enough.)
+
 Testing
 ~~~~~~~
 
@@ -43,6 +59,10 @@ this will do the job:
 (Not all tests require all of these, but it's safer to include all of
 them.)
 
+Secondly, be aware that if you want to test modifications to the
+source files, you will need to run `make watch`, or `make` before each
+test run as explained above.
+
 Now running an individual test is as simple as:
 
     perl t/chkstow.t

Makefile.am

@@ -286,3 +286,28 @@ ChangeLog: doc/ChangeLog.OLD
 	else \
 	    echo "Not in a git repository; can't update ChangeLog."; \
 	fi
+
+# Watch for changes, and if any rebuilds are required, also do a
+# make install.
+#
+# If we solved https://github.com/aspiers/stow/issues/84, we could
+# probably ditch this:
+watch:
+	@echo "Watching for changes to program source files ..."
+	@while true; do \
+	    if $(MAKE) 2>&1 | \
+		grep -vE 'make\[[1-9]\]: (Entering|Leaving) directory ' | \
+		grep -v 'Nothing to be done'; \
+	    then \
+		echo; \
+		echo "-----------------------------------------------------"; \
+		echo "make found things to rebuild; doing $(MAKE) install ..."; \
+		echo; \
+		$(MAKE) install; \
+		echo; \
+		echo ">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>"; \
+		echo; \
+	    fi; \
+	    sleep 1; \
+	done 2>&1 | \
+	grep -vE 'make\[[1-9]\]: (Entering|Leaving) directory '

NEWS

@@ -15,6 +15,11 @@ News file for Stow.
 
 ***** Added a CONTRIBUTING.md file
 
+***** Add a =watch= target to =Makefile=
+
+      =make watch= provides easy continual pre-processing during
+      development, which reduces the risk of debugging the wrong code.
+
 ***** Removed texinfo.tex from the distribution
 
       This eliminates existing and future bit-rot.