Documentation: allow installing man pages to a user defined directory

Documentation/DocBook/Makefile hard codes the prefixed path to which you
can install the built man pages (/usr/local prefix). That's unfortunate
since the user may want to install to another prefix or location (for
example, a distribution packaging the man pages may want to install to a
random temporary location in the build process).

Be flexible and allow the prefixed path to which we install man pages to be
changed with the INSTALL_MAN_PATH environment variable (and use the same
default as other similar variables like INSTALL_HDR_PATH).
Signed-off-by: Herton R. Krzesinski <herton@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DocBook/Makefile

@@ -61,11 +61,14 @@ MAN := $(patsubst %.xml, %.9, $(BOOKS))
 mandocs: $(MAN)
 	find $(obj)/man -name '*.9' | xargs gzip -nf
 
+# Default location for installed man pages
+export INSTALL_MAN_PATH = $(objtree)/usr
+
 installmandocs: mandocs
-	mkdir -p /usr/local/man/man9/
+	mkdir -p $(INSTALL_MAN_PATH)/man/man9/
 	find $(obj)/man -name '*.9.gz' -printf '%h %f\n' | \
 		sort -k 2 -k 1 | uniq -f 1 | sed -e 's: :/:' | \
-		xargs install -m 644 -t /usr/local/man/man9/
+		xargs install -m 644 -t $(INSTALL_MAN_PATH)/man/man9/
 
 # no-op for the DocBook toolchain
 epubdocs:
@@ -237,7 +240,9 @@ dochelp:
 	@echo  '  psdocs          - Postscript'
 	@echo  '  xmldocs         - XML DocBook'
 	@echo  '  mandocs         - man pages'
-	@echo  '  installmandocs  - install man pages generated by mandocs'
+	@echo  '  installmandocs  - install man pages generated by mandocs to INSTALL_MAN_PATH'; \
+	 echo  '                    (default: $(INSTALL_MAN_PATH))'; \
+	 echo  ''
 	@echo  '  cleandocs       - clean all generated DocBook files'
 	@echo
 	@echo  '  make DOCBOOKS="s1.xml s2.xml" [target] Generate only docs s1.xml s2.xml'