Add documentation for the DSUB option

Author: drdv

README.md

@@ -94,7 +94,7 @@ are given in `{...}`, `(.)` shows the default)
 
 + `OUTPUT_FORMAT`: `{(ANSI), HTML, LATEX}`
 + `EXPORT_THEME`: see [Export to HTML and Latex](#export-to-html-and-latex)
-+ `SUB`: see [Substitutions](#substitutions)
++ `SUB`, `DSUB`: see [Substitutions](#substitutions)
 + `TARGETS_REGEX`: regex to use for matching targets
 + `VARIABLES_REGEX`: regex to use for matching variables
 * `VARS`: `{0, (1)}` show documented variables
@@ -165,6 +165,19 @@ shown. The supported parameters are:
 See the `Makefile` of this project and the test recipes in
 `test/recipes/test-substitution-*` for examples.
 
+The option `DSUB` performs substitutions in the original descriptions. The expected
+format is `NAME:VALUES`. For example, using `-v DSUB='$$(DEPS):$(DEPS)'`, the target
+
+```Makefile
+DEPS := x y
+## Prerequisites: $(DEPS)
+t: $(DEPS)
+````
+would be documented as:
+````
+t    Prerequisites: x y
+````
+
 ## Export to HTML and Latex
 
 We use

makefile-doc.awk

@@ -5,7 +5,7 @@
 #  Author: Dimitar Dimitrov
 # License: Apache-2.0
 # Project: https://github.com/drdv/makefile-doc
-# Version: v1.5
+# Version: v1.6
 #
 # Usage (see project README.md for more details):
 #   awk [-v option=value] -f makefile-doc.awk [Makefile ...]
@@ -24,6 +24,7 @@
 #   + OUTPUT_FORMAT: {(ANSI), HTML, LATEX}
 #   + EXPORT_THEME: see below
 #   + SUB: see below
+#   + DSUB: see below
 #   + TARGETS_REGEX: regex for matching targets
 #   + VARIABLES_REGEX: regex for matching variables
 #   * VARS: {0, (1)} show documented variables
@@ -77,6 +78,18 @@
 #   + I       initial string, e.g., {
 #   + T       termination string, e.g., }
 #
+# DSUB:
+#  Similar to SUB but performs substitutions in the original descriptions. The expected
+#  format is NAME:VALUES. For example, using -v DSUB='$$(DEPS):$(DEPS)', the target
+#
+#  DEPS := x y
+#  ## Prerequisites: $(DEPS)
+#  t: $(DEPS)
+#
+#  would be documented as: t   Prerequisites: x y
+#
+#  Values containing newline chatacters are not supported.
+#
 # Code conventions:
 #   * All local variables in functions should be defined in the function signature (awk
 #     is a bit special in that respect).