core: Makefiles - catch unavailbility of docbook2man tool

- print an info message instead of ending up in execution error of
Makefile rule trying to generate man pages from xml files
- DOCBOOK var renamed to DOCBOOKMAN to be more suggestive for its
purpose
- related to GH #1624

src/Makefile.modules

@@ -278,16 +278,17 @@ endif
 
 #man page build rules
 ifneq (,$(wildcard $(MOD_NAME).xml))
-
+ifneq ($(DOCBOOKMAN),)
 $(MOD_NAME).7: $(MOD_NAME).xml
-	$(DOCBOOK) -s ../../../doc/stylesheets/serdoc2man.xsl $<
+	$(DOCBOOKMAN) -s ../../../doc/stylesheets/serdoc2man.xsl $<
 
 man: $(MOD_NAME).7
-
 else
-
 man:
-
+	@echo "info - no docbook to man tool to convert $(MOD_NAME).xml"
+endif
+else
+man:
 endif
 
 

src/Makefile.shared

@@ -43,8 +43,8 @@ sh_err_fail=
 try_err=$(1) ; if [ $$? != 0 ]; then echo ERROR: $(1) failed ; fi
 endif
 
-DOCBOOK = $(shell which docbook2x-man 2>/dev/null)
-ifeq ($(DOCBOOK),)
-	DOCBOOK = $(shell which db2x_docbook2man 2>/dev/null)
+DOCBOOKMAN = $(shell which docbook2x-man 2>/dev/null)
+ifeq ($(DOCBOOKMAN),)
+	DOCBOOKMAN = $(shell which db2x_docbook2man 2>/dev/null)
 endif
 

src/Makefile.utils

@@ -117,16 +117,17 @@ endif
 
 #man page build rules
 ifneq (,$(wildcard $(UTIL_NAME).xml))
-
+ifneq ($(DOCBOOKMAN),)
 $(UTIL_NAME).7: $(UTIL_NAME).xml
-	$(DOCBOOK) -s ../../doc/stylesheets/serdoc2man.xsl $<
+	$(DOCBOOKMAN) -s ../../doc/stylesheets/serdoc2man.xsl $<
 
 man: $(UTIL_NAME).7
-
 else
-
 man:
-
+	@echo "info - no docbook to man tool to convert $(UTIL_NAME).xml"
+endif
+else
+man:
 endif
 
 $(man_prefix)/$(man_dir)/man8: